Introduction to Earth Science

Chapter 12 - Streams, Rivers, and Water Underground

This chapter is about the impact of flowing water on the surface of the earth and underground. Flowing water is a primary force affecting changes to the landscape. Understanding how water interacts with the landscape and materials underground are essential to managing water resources and protection of people and property from the effects of floods, droughts, and water contamination. The Hydrologic Cycle diagram (Figure 12-1) illustrates the atmospheric interactions of water involving evaporation (producing water vapor), condensation (clouds), and precipitation (as rain, snow, and ice). One on the ground, melting ice and rain either evaporates, soaks into the ground, is absorbed by plants, runs off (into streams), or sinks into the ground (becoming groundwater). Water in streams or underground moves under the influence of gravity, ultimately flowing into the oceans or trapped in inland lake basins. Humans now influence the flow of water in large regions of the world, profoundly impacting the environment. Water cycle
Fig. 12-1. The Hydrologic Cycle (or Water Cycle)

Hydrology and Geohydrology

Hydrology is the science concerned with the properties of the earth's water, especially its movement in relation to the land's surface. The study of water underground (called groundwater) is geohydrology. Water can sink into the ground and rise again to the surface over and over again as it flows from its source areas in upland regions to where it flows into the sea.

The Hydrosphere encompasses all the waters on the Earth's surface, such as oceans, lakes, rivers, and streams. Earth's oceans cover about 70% of the surface and contains 97.25% of the water. It also includes "ice" (glaciers are discussed in Chapter 12). The table below illustrates where water occurs on Earth.

Reservoir Volume / 106, km3 Percent of total


1370 97.25

Ice caps and glaciers

39 2.05

Deep groundwater

5.3 0.38

Shallow groundwater

4.3 0.30
Lakes 0.125 0.01
Soil moisture 0.065 0.005
Atmosphere 0.013 0.001
Rivers 0.0017 0.0001
Biosphere 0.0006 0.00004
Total 1408.7 100

Atmospheric Interactions in the Hydrologic Cycle include:

evaporation—the physical conversion of a liquid into a vapor.

transpiration—The process by which water in plants is transferred as water vapor to the atmosphere.

—the conversion of atmospheric water vapor into liquid water or ice (causing precipitation).

precipitation—in meteorology usage it means "rain, snow, sleet, or hail that falls to the ground." (In chemistry usage: precipitation means "the action or process of precipitating a substance from a solution, such as salt precipitating from evaporating seawater.")

rainshadow effect—the downwind side of a mountain range (or high volcano) that partially blocks the flow of moist air, forcing precipitation on the prevailing windward side, and creates more arid conditions on the downwind side. Rainshadow effects are illustrated in Figures 12-2 and 12-3 for the state of California.
California water cycle
Fig. 12-2. California landscape and weather patterns influence precipitation.
California precipitation
Fig. 12-3. Map of California precipitation and streams. Note the impact of California's mountain ranges on the location of desert regions (show in orange and red). The prevailing wind direction (carrying moisture-bearing clouds) is generally from west to east. The east side of mountain ranges in California tend to be rainshadow areas.

Comparison of Freshwater to Seawater

"Freshwater" is a general term for water that does not contain significant amounts of salt. Freshwater collected directly from rain and snow melt contains traces of salts derived from evaporating seawater, atmospheric dust, and pollution. Water in streams, ponds, lakes, and rivers are generally considered freshwater, although some inland lakes and stream in isolated desert basins can be even saltier than seawater. The natural weathering of rocks and soils contributes salts to freshwater. Most rivers pick up increasing amount of salt along their route to the sea. Through geologic time, salts derived from land contribute to the saltiness of sea water, and the salt is concentrated by the evaporation of seawater. Large quantities of salts in seawater are absorbed by saturated sediments and crustal rocks on the seafloor, which, in turn, are subducted and eventually return to be a part of the lithosphere.
Chemical components of seawater and freshwater
Fig. 12-4. Composition of dissolved components in seawater and freshwater in streams

What’s the difference between a stream and a river?

A stream is a small flow of water in a channel or bed, whereas a river is a collection of many streams. Other names for a stream include a brook, creek, rivulet, or small river.

There have been various ways to classify rivers and streams. One method called the Strahler Stream Order (Arthur Strahler, 1957) divides streams in a hierarchy similar to branches on a tree, with 1st order streams akin to twigs on branches, and the highest order being the trunk of a tree (Figure 12-5). Most 1st order streams are in the headwater regions of a drainage basin, Where 1st order steams combine they become 2nd order streams. Where 2nd order streams merge they become 3rd order streams. 3rd order streams merge to for 4th order, and so on. However, lower order streams merging with a higher order stream do not change the order the higher stream (Figure 12-6 and 12-8). The progression size continues until a “river” begins at the confluence of principle stream tributaries. (A tributary is a stream that discharges into a larger river or lake; the location where they merge is called a confluence; example Figure 12-9.) The index of a stream or river may range from 1st order (having no tributaries) to large orders, with examples the Ohio River being a 8th order that discharges into the Mississippi River, a 10th order. The largest river, the Amazon, is a 12th order.

To qualify as a hydrological feature called a “stream” the water flow must be perennial (meaning flows year round) or recurring (meaning it flows for at least part of a year). For instance, many streams in drier regions of the western United States only flow during wet periods such as in the spring after snow melts, or after periods of summer monsoonal rains. During drought periods, some streams may not flow for years.

Both rivers and streams are bodies of flowing water, however, in general, rivers are larger, deeper, and longer than streams. A simplest definition might be that "you can wade across a stream, but you would have to swim across a river." However, by this definition many streams become rivers during periods of flood, and rivers turn into streams during dry periods.

Strahler stream order classification
Fig. 12-5. Classification of stream order (smallest to largest) after Strahler (1957)
Loma Prieta Peak near San Jose, CA
Fig. 12-5. 1st order stream drainages in the headwater region of the Guadalupe River watershed on Loma Prieta Peak in San Jose California.

Worlds Largest Rivers

River Name Length (km) Length (miles) Drainage area (km²) Average discharge (m³/s) Outflow
Amazon 6,992 4,010 6,915,000 175,000 Atlantic Ocean
Ganges - Brahmaputra - Meghna 2,948 1,832 1,635,000 42,470 Bay of Bengal
Congo 4,371 2,716 3,822,000 41,200 Atlantic Ocean
Yangtze (Chang Jiang) 6,380 3,964 1,940,000 35,000 East China Sea
Orinoco 2,140 1,330 880,000 33,000 Atlantic Ocean
Paraná (Río de la Plata) 3,998 2,484 3,100,000 25,700 Atlantic Ocean
Yenisei 5,550 3,449 2,580,000 18,040 Kara Sea
Lena 4,260 2,647 2,490,000 16,200 Laptev Sea
Mississippi 6,270 3,896 2,980,000 16,200 Gulf of Mexico
Mekong 4,023 2,500 811,000 14,800 South China Sea
Ganges 2,510 1,560 907,000 13,159 Padma River
Ayeyarwady 2,170 1,348 411,000 13,000 Andaman Sea
Amur (Heilong) 4,352 2,714 1,855,000 11,400 Sea of Okhotsk
Ob' - Irtysh 5,410 3,449 2,990,000 10,300 Gulf of Ob
Mackenzie - Peace - Finlay 4,241 2,635 1,790,000 10,300 Beaufort Sea
Saint Lawrence 3,058 1,900 1,030,000 10,100 Gulf of Saint Lawrence
Niger 4,167 2,589 2,090,000 9,570 Atlantic Ocean
Volga 3,692 2,294 1,380,000 8,060 Caspian Sea
Zambezi 2,574 1,599 1,331,000 7,070 Indian Ocean
Indus 3,200 2,000 1,165,000 6,600 Arabian Sea
Nile 6,650 4,132 3,349,000 5,100 Mediterranean Sea
Western US Rivers
Columbia 2,250 1,450 415,211 7,500 Pacific Ocean
Colorado 2,333 1,450 390,000 1,200 Pacific Ocean
Rio Grande 3,057 1,900 570,000 82 Gulf of Mexico
Sacramento River 716 445 71,000 820 Pacific Ocean

1st order stream in Great Basin National Park, Nevada
Fig. 12-6. A 1st order stream begins at a melting snow field in Great Basin National Park, Nevada.
2nd order stream - Lehman Creek in Great Basin National Park, Nevada
Fig. 12-7. A 2nd order stream (Lehman Creek) in Great Basin National Park.
3rd order stream, San Benito River, Hollister, California
Fig. 12-8. A 3rd order stream. (San Benito River) is dry (ephemeral or recurring) during summer months between storms. During flood, water covers the entire stream bed.

Geographic factors that control the flow of water

Gravity pulls water down slope. If it flows on the surface as runoff, or underground as groundwater. Groundwater discharges into streams even during periods when precipitation is not providing runoff.

A drainage basin
is a region drained by a principle stream extending from upland headwater regions down to where the stream merges with another body of water (a larger stream or river, lake, or ocean). A divide is a line that marks the boundary between two drainage basins.

Stream flow
is the amount of water moving downstream. The measure of the amount of water associated with a stream or river is called stream discharge. Stream discharge is the volume of water to pass a given point on a stream bank per unit of time, usually expressed in cubic meters of water per second. The discharge is measured by cross sectional volume of the channel times the velocity of moving water and sediment in the water and along the bottom of the stream bed.

A stream gradient is the grade (slope) measured by the ratio of drop in a stream per unit distance, usually expressed as feet per mile or meters per kilometer (Figure 12-10).

Stream Profiles In most cases, the profile of a river drainage usually decreases gradually downstream. 1st, 2nd, and 3rd order stream have a high stream gradient and are fast-flowing and characterized by rapids and waterfalls. As rivers descend out of mountainous regions, the streams slow down and grow deeper and wider, and their channels gradually move back and forth across and ever-widening floodplain. Large quantities of groundwater move into and out of sediments on floodplains and tend to stabilize water flow in the lower river basins (such as along the Mississippi River downstream of Memphis where the floodplain is many miles wide; Figure 12-11).
Fig. 12-10. Profile of a river drainage. The stream gradient for most rivers decreases downstream.

Confluence of the Green and Colorado Rivers in Canyonlands National Park, Utah
Fig. 12-9. Confluence of the Green River (left) and Colorado River (right) in Canyonlands National Park, Utah. The Colorado River changes from 4th to 5th order at the confluence and continues downstream.
Mississippi River at Memphis, Tennesee
Fig. 12-11. Mississippi River at Memphis is already a 9th order stream.

How do streams transport and deposit sediments?

Streams move downslope under the influence of gravity, the passage of water is called stream flow. Several factors control the amount of sediment that can be carried by a stream: 1) volume of stream flow, 2) the stream gradient, 3) shape of the stream channel, and 4) kinds and volume of sediments available for erosion in a drainage basin. During floods, the volume and rate of stream flow increases, and erosion along the stream bed mobilizes sediments that accumulate during times of decreasing stream flow. Erosion carves the sides of stream channels, contributing sediments to streams and allowing the channel to migrate over time. Turbulence in the often violent or unsteady movement and mixing of air or water, or of some other fluid.; a most important factor influencing sediment transport in a stream.

A flood is an overflowing of a large amount of water beyond its normal confines—inundating areas that are typically dry at least part of the year). Floods are linked to seasonal precipitation patterns (such as monsoons or spring snow melt) or catastrophic events (such as intense rainstorms, collapse of natural or manmade dams or levees).

Sediments are moved by streams in different ways (Figure 12-15). Fine grained particles and dissolved components are carried in suspension (called the suspended load). With increasing turbulence, the suspended load increases. Particles that are to heavy to be carried in suspension roll, bounce, and hop along the stream bed; this process is called saltation. This moving mass of solid material is called the bed load. During floods it is often possible to hear the roar created by rocks cracking into each other as they tumble along the stream bed.

The shape of a stream channel and the stream gradient controls the amount of sediment that can be transported down stream. In straight channels, stream water moves as laminar parallel vectors, but with increasing speed and when objects hinder flow, the water becomes turbulent, constantly mixing. With increasing speed and turbulence stream water can carry more sediment (and larger particles) is suspension and as bedload.
Gravel Bar Floodwaters
Fig. 12-13. A gravel bar along a mountain stream. Large boulders and gravel moves in times of flood. Fig. 12-14. Floodwaters are brown because they contain high concentrations of fine-grained sediments carried in suspension.
how sediments move in a stream
Fig. 12-15. Turbulent stream flow transports sediments by suspension (including dissolved materials and fine sediments). Coarser sediments roll, bounce, and hop along the stream bed in a process called saltation. This is called bedload.
Stream shape
Fig. 12-16. Cross section of a straight stream showing the effects on stream flow capacity and velocity.

Meandering creates winding river channels

A meander is a bend in a sinuous watercourse. Meandering is the process when the faster-moving water in a river erodes the outer banks and widens its valley, and the slower-moving water on the inner side of the bend becomes a place where sediments are deposited (point bars). As a result, rivers tend to constantly change their course over a floodplain over time (Figures 12-17 and 12-18).

An oxbow is a crescent lake on a stream floodplain formed when a meandering stream channel is cut off and isolated by changes in a stream channel.
Meanders Meandering stream
Fig. 12-17. Stream channel flow results in meandering forming cutbanks and point bars (see Fig. 12-20 below). Fig. 12-18. Example of a meandering river channel with cutoff channels and oxbows.

Parts of a river system

The headwaters is the source region of primary precipitation catchment, including snow melt and other surface runoff, and groundwater discharge from springs. A precipitation falls (and melts) the water move downslope, collecting into a more cohesive flow. Over time, this flow carves gullies. A gully is a water-worn ravine or trench-like depression where water runs off the a hilly or mountainous landscape, caused by erosion.

In upland regions where 1st to 3rd order streams dominate the landscape, waterfalls and rapids are common (Figure 12-19). A waterfall is a cascade of water falling from a height, formed when a river or stream flows over a precipice or steep incline. A rapid is a fast-flowing and turbulent part of the course of a river or stream where a change in gradient and shape of the channel changes, commonly associated with gravel bars or obstructions (bedrock and boulders) in the river bed.

A tributary is a stream that flows into another stream or river (rather than a lake or sea). Tributaries combine to be a part of a larger drainage basin.

The river channel is the path of a river; the size and shape of a channel depends on the volume of water flowing in it, the gradient of the stream, and the nature of rock and sediments exposed in the river bed.

Parts of river valleys where the stream gradient diminishes, instead of carving downward into the river bed, stream meander back and forth, broadening the valley at the expense of the hillsides along the drainage, allowing floodplains to develop. A floodplain is a flat, low-lying area along a river that gets covered with water when the river floods, overflowing its natural levee. Slow moving storm water drops fresh sediment on floodplains.

The riverbank is land immediately exposed along a river during non-flood conditions, but sculpted by flowing water during high water conditions. River banks can include cutbanks (stream-cut exposures of bedrock and older stream sediments), natural levees (buildups of sediments along the path of the river channel deposited by floodwaters), gravel bars, and point bars.

Many river floodplains have wetlands. Wetlands are lowland areas where water covers the surface, at least part of the year. Also called swamps, marshes, or bogs. Wetlands provide habitat to many varieties of plants and animals.

A splay is a small fan-shaped or outspread alluvial deposit formed where an overloaded stream breaks through a levee (artificial or natural) and deposits its material (often coarse-grained) on the floodplain (Figure 12-22). Like meandering, the formation of splays contributes sediments to flood plains, particularly during times of flood.

The mouth is where a river or stream enters a larger body water (lake or ocean). Deltas form at the mouths of rivers.

A delta is an accumulation of sediments at the mouth of a river that may consist of a network of distributary channels, wetlands, bars, tidal flats, natural levees and beaches that typically shift from on location to another. Delta shape is dependent of dominant current conditions where the mouth of the river: tide-, sea wave-, and storm-dominated.

A distributary is the opposite of a tributary. A distributary channel is a stream that branches off and flows away from a main stream channel. Distributaries are a common feature of river deltas where slowing water and interaction of ocean tides, waves, and currents influence the distribution of sediments along a coastline. Distributaries can form along floodplains near the mouths of streams entering larger rivers or lakes.

Grand Canyon of the Yellowstone
Fig. 12-19. Waterfalls and rapids are common features in headwater regions. Upland stream and river valleys typical have a V-shape.
Floodplain environments
Fig. 12-20. River features
Salinas River
Figure 12-21. Point bars (gravel bars) along the Salinas River in Monterey County, California.
Fig. 12-22. Splays and distributary channels on a river delta.

Examples of River Deltas

Amazon Delta Nile Delta Lena Delta, Siberia Indus River Delta
Fig. 12-23. Amazon River Delta Fig. 12-24. Nile River Delta Fig. 12-25. Lena River Delta, Siberia Fig. 12-26. Indus River Delta
Yellow River Delta, China Mississippi Delta Mississippi Birdfoot Delta Colorado River Delta
Fig. 12-27. Yellow River Delta Fig. 12-28. Changes to the Mississippi River Delta over the last 4000 years. Fig. 12-29. The Mississippi Birdfoot Delta is largely controlled by human activities. Fig. 12-30. A river no more. Very little water makes it to the Colorado River Delta.


Types of drainage networks and drainage patterns.

A drainage pattern is a pattern created by stream erosion over time that reveals characteristics of the kind of rocks and geologic structures in a landscape region drained by streams (Figure 12-31).

Dendritic drainage patterns are randomly shaped systems that commonly form on heterogeneous bedrock, such as flat-lying sediments or sedimentary rocks where there is nothing to influence the flow of water.

Parallel drainage patterns
develop on inclined surfaces such as gently dipping strata.

Trellis drainage pattern are common in terrain with folded bedding (parallel series of anticlines and synclines).

Rectangular drainage patterns are common in fractured and faulted hard bedrock (usually igneous and metamorphic rocks).

Deranged drainage patterns
occur in areas that geologic events have disrupted the landscape. Example of deranged drainages occur in recently glaciated landscapes and in active volcanic regions.

Radial drainage patterns are associated with circular-shaped basins and domes.

drainagesFig. 12-31. Stream drainage patterns can reflect the structure and composition of bedrock.


Sediment Supply versus Stream Gradient

A river or stream is rarely in a state of equilibrium when it comes to erosion and deposition processes. In general, upland regions receive the greatest amount of precipitation, and are places where sediments are being generated. If tectonic forces are causing the land to rise, stream gradients increase, and streams carve deeper into their valleys, forming canyons (Figure 12-32).

Where streams loose their gradient, they slow down and sediments are deposited, filling in their valleys. Braided streams are common in landscapes where stream valleys are overloaded with sediment. Streams dropping their sediments are filling their channels, causing the channels to constantly migrate across a floodplain (Figure 12-33). Changes in climate can cause changes in the amount of sediments contributed to a stream flood plain.
Grand Canyon
Fig. 12-32. Colorado River in the Grand Canyon, Arizona
. Canyons form where stream erosion dominates over deposition.
Braided Stream
Fig. 12-33. Braided streams
form in stream valleys overloaded with available sediments, back filling valleys with sediments.
Changing Base Levels and the Formation of Stream Terraces

Through time, stream valleys are constantly changing. One stream drainage may grow at the expense of the ones adjacent to it.

Base level
is the lowest level to which a land surface can be reduced by the action of running water. It is typically equivalent to the lowest point a river or stream can reach entering the ocean or other large body of water. Base level of a stream entering the ocean will rise and fall with changes in sea level.

A stream terrace is one of a possible series of level surfaces on a stream valley flanking and parallel to a stream channel and above that marks the level of a floodplain in the geologic past. A change in stream gradient or stream flow can cause streams to carve into their floodplain, leaving a step like terrace along the side of the valley. Multiple events related to these changes can leave a series of step-like terraces (old floodplain surfaces) preserved along a stream valley. The erosional downcutting of a stream into its floodplain is called stream rejuvenation.
terraces Arroyo Seco
Fig. 12-34. Terraces along Arroyo Seco Canyon in central California reflect landscape change including uplift of the Coast Range and changes caused by sea level changes along the coast caused by the ice ages. When sea level rises, the streams back fill their valleys with sediments and form floodplains. When sea level falls the stream gradient increases, and streams carve into their valleys.
Goosenecks of the San Juan
Fig. 12-35.
The "Goosenecks of the San Juan River" in New Mexico is an example of a rejuvenated stream valley. The river was originally a meandering stream on a broad floodplain. Uplift in the region (or other causes) caused a change in stream gradient, causing the river to cut into its meandering channel, creating the unusual shape of the San Juan River canyon.

Fig. 12-33. Formation of stream terraces along a rejuvenated stream valley. Climate changes, sea level changes, and tectonic uplift can cause streams to cut into their floodplains producing abandoned stream terraces.

Floods don't have to be disasters

Floods are a natural cycle of all stream and river drainages. During times of flood, rivers overflow their banks and levees and pour sediment-laden water onto low lying areas, providing new sediments (and nutrients of plants) to their floodplains. Figure 12-36 is a airliner view of a river in flood. The natural levees are cover with trees and stand out avoid the flooded floodplain. One of the most important geologic principles is "never build on a floodplain" ...a rule that has failed repeatedly throughout history.
flooded levee
Fig. 12-36. A river in flood.
Sediment-laden waters cover the entire floodplain.
Pakistan 2010 flood
Fig. 12-37. The 2011 flood in Pakistan
caused major devastation of a large region, impacting millions.
Grand Forks, ND 2007
Fig. 12-38. 2009 Flood in Grand Forks, North Dakota.
Winters with heavy snow lead to spring floods.

Great Flood Disasters in History

Year Flood Area Death Toll Comments
1931 Huang He (Yellow) River, China 1,000,000 to 3,700,000 The "Yellow" river gets its name because of its high silt content. The silt contributes to be buildup of its expansive floodplain.
1887 Huang He (Yellow) River, China 900,000 to 2,000,000  
1938 Huang He (Yellow) River, China 500,000 - 900,000 This flood was cause by Chinese military forces cutting through levees in order to flood area occupied by Japanese invasion forces. It worked, kinda.
1642 Huang He (Yellow) River, China 300,000 This flood was cause by rebel forces cutting through dikes along the city of Kaifeng.
1975 Ru River, Banqiao Dam, China 230,000 The collapse of the Banquia Dam, along with others, following a heavy rain caused by a typhoon. was the worst dam-related disaster in history.
1931 Yangtze River, China 145,000 Historical records show that Yangtze has had more than 1,000 recorded floods in Chinese history.
1099 Netherlands and England 100,000 This disaster was caused by a combination of high tides and storms causing flooding in low-lying areas.



Groundwater is water beneath the land's surface, filling pore spaces in saturated soil and rock. It is water that supplies wells and springs.

Whereas hydrology is the science concerned with the properties of the earth's water, especially its movement in relation to the land's surface. The study of groundwater is called geohydrology (or hydrogeology).

Where does water occur in the subsurface?

Geologists use the terms "aquifer, aquitard, and aquiclude" to describe the character of rocks in the subsurface in relation to whether or not they can contain or provide a supply of groundwater (Figure 12-39).

An aquifer is a porous and permeable rock or sediment layer, such as a sand or sandstone, containing groundwater that can be used to supply wells.

An aquitard is a zone or layer of low permeability adjacent to an aquifer; the permeability is so low it cannot transmit any useful amount of water. Aquitards act as confining layers to confined aquifers. Confined aquifers can be used as water storage.

An aquiclude is an impermeable body of rock or stratum of sediment, or an impermeable fault zone, that acts as a barrier to the flow of groundwater.

Groundwater illustrated
Fig. 12-39.
Features associated with water underground

Map of groundwater regions in the United States
Fig. 12-40.
Groundwater map of the US

Porosity and Permeability

What is the difference between porosity and permeability and how these relate to the movement of groundwater?

is the state of being porous, or the ratio of the volume of all the pores (gas- or fluid-filled space) in a material to the volume of the whole.

Permeability is a measure of the ability of a porous material (rock or unconsolidated sediments) to transmit fluids.

Rocks and sediment have very different characteristics in regard to porosity and permeability (Figure 12-41). For instance:
* Sand and gravel have both high porosity and high permeability (sandstone and gravel may be less so because much of the pore space is filled in with cement).
* Mud and shale have high porosity but very low permeability (because fine-grained particles impede the flow of water).
* Fracture rocks like granite or volcanic and metamorphic rocks may have low porosity, but the fractures allow water to flow, so they may have high permeability.
Porosity and Permeability
Fig. 12-41.
Examples of porous and permeable materials.

Saturated versus Unsaturated Zones

The water table is the level below which the ground is saturated with water. The water table is influenced by the gravitational flow of water underground, typically following the general topography of a landscape, but can be changed by the extraction of water from a well, or construction of a dam. The groundwater table rises and falls with changes in seasonal precipitation.

The phreatic zone is the zone of saturated rock or sediment below the water table where pore spaces between grains or within fractures are mostly filled with water (also called zone of saturation).

The vadoze zone
(also called the unsaturated zone) is the typically shallow subsurface interval between the land surface and the underlying phreatic zone or zone of saturation. The vadose zone extends from the top of the ground surface to the water table.

In many places there is a zone called the capillary fringe. Capillary fringe is a subsurface layer above the water table in which groundwater seeps up from a water table by capillary action to fill pores.
Vadose and Phreatic zones
Fig. 12-41.
Water table is the boundary between the saturated phreatic zone and unsaturated vadose zone. A zone called a capillary fringe exists in many place just above the water table.

Springs and Wells

A spring is any natural occurrence where water flows to the surface of the earth from below the surface. Springs typically occur in locations where the water table in an aquifer meets the ground surface, allowing water to discharge onto the surface or into a body of water (such as a stream, lake, or the ocean). In arid regions, springs are commonly revealed by an abundance of plant growth in the vicinity where water is at or near the surface.

A well is simply a hole dug or drilled in the ground that descends through the water table for the purpose of extraction of fluids (water, oil, and gas). Wells in unconfined aquifers will fill with water to the level of the water table.
An artesian well is a well drilled through impermeable strata to reach water in a confined aquifer capable of rising to the surface by internal hydrostatic pressure. In geohydrology the word "head" is used to describe the height that groundwater will rise in a confined well under artesian water pressure.

The effects of water extraction from a well

Pumping water out of the ground faster that groundwater can refill the pore spaces in the saturated zone will cause the water table to fall. The drawdown of a well is a reflection of the reduction in the volume of water in an aquifer. Drawdown is also used to describe the reduction in the volume of water in a lake or reservoir. In many places a lake or pond is just part of a larger reservoir involving water on the surface and underground.

When water is pumped from an unconfined aquifer it will create a cone of depression. A cone of depression is an inverted cone-like depression in the water table caused by groundwater extraction in a well in an unconfined aquifer. When too much pumping takes place, the cone of depression will sink to the base of a well, and it will run dry. When a well is shut off, the cone of depression will rise again as water returns to the depleted zone. Unfortunately in many dry regions around the world, wells keep running dry, so people drill deeper wells. In many places, this unsustainable drawdown not only "mines" the water, but can steal water from other wells nearby (Figure 12-43). In some places, the groundwater supply has been so depleted that the water table has sunk hundreds or even thousands of feet from its original level. There are many negative side effects (discussed below).
Types of wells
Fig. 12-42. Springs, wells and groundwater
Artisian well at McAlpine Lake near Gilroy ,California
Fig. 12-43.
This artesian well flows into McAlpine Lake near Gilroy, California.Drawdown from a well forms a cone of depression
Fig. 12-44.
A Cone of depression forms from the drawdown of a well by pumping out groundwater.

Influent and Effluent Streams

Changes in the level of a water table can influence whether water sinks into the ground or flows to the surface. An
influent stream
is a stream that contributes water to the zone of saturation of groundwater and develops bank storage.

An effluent stream is a stream that is fed by groundwater seeping to the surface.

When large quantities of water are pumped from an unconfined aquifer a stream flowing across the area may loose some or all of its water into the subsurface if the drawdown of the water table is significant, and the ground beneath the stream consists of highly permeable sediments or rocks.
Influent and Effluent streamsFig. 12-44. Influent and effluent streams react to levels in the water table relative to the surface level of a stream.

Problems caused by over-pumping groundwater

Subsidence is the lowering of the land surface. Subsidence also occur in areas where extensive extraction of water or petroleum allows pore spaces between grains or fractures in rock to collapse, reducing volume, and resulting in subsidence of the land's surface. Although subsidence can be caused by tectonic forces, in many places, too much groundwater extraction is causing the surface of the land to sink. Classic examples include what is happening to Houston, Texas, but subsidence is occurring anywhere where well water is used in excess of recharge (Figure 12-45).

Over pumping harms wildlife. Lower water tables prevent water from draining from springs or shuts down stream flow that would otherwise be available as a water source for wildlife.

Salt water intrusion ia the contamination of groundwater by seawater caused by too much well water withdrawal from an unconfined coastal aquifer (Figure 12-46).
Houston subsidence from groundwater withdrawal
Fig. 12-45.
Subsidence in the Houston-Galveston area
saltwater intrusion
Fig. 12-46. Saltwater intrusion
caused by over pumping

Water Pollution From Point and Non-Point Sources

Freshwater on the surface (rivers, lakes, and streams) and underground can become polluted from a variety of ways; they are lumped together in two groups: point and non-point sources.

Point sources are places where pollutant materials originate from a single known locality, such as pollution from the effluent of a poorly designed sewage treatment plant, a landfill, or a spill associated with a hazardous waste site. Point sources can typically be contained (or better prevented in advance).

Non-point sources of pollution are more problematic because their origin is not from an easily identifiable source. Examples include acid rain created by industrialization, oil and other waste from automobiles, air pollution, pesticides and fertilizers used in agriculture can become widely distributed by the wind. DDT, a pesticide widely used in the 1950 and 1960s was finally banned in the United States in 1972, but it can still be found in trace amounts throughout the environment today.
Groundwater contamination
Fig. 12-47. Point source and non-point source pollution of streams and groundwater.

Caves, Caverns, and Karst

A cavern is an underground passage formed by dissolution of rock (typically limestone) by flowing groundwater. The word cave is often used as a substitute for the more correct term cavern. Caves can be simple sheltered areas beneath a large rock overhang. The name karst is applied to regions of irregular terrain underlain by limestone and is characterized by numerous caves, sinkholes, fissures, and underground streams. Figure 12-48 is an example of karst in China.
Karst landscape
Fig. 12-48.
Karst landscape near Qifeng, south of Guilin, Guangxi (China) formed from the dissolution of limestone rock formations.
Carbonate chemical reactions
Fig. 12-49.
Chemistry of calcium carbonate reactions
in the environment. Calcite dissolves in freshwater and precipitates from seawater.
Carbonate Depositional Environments
Fig. 12-50.
Carbonate depositional environments occur in shallow warm marine settings, commonly associated with reefs.
Limestone is host to most caverns (they occur in other rocks, but by comparison they are small and rare). Limestone is composed dominantly of the mineral calcite, CaCO3. Carbon dioxide dissolved in water forms weak carbonic acid which reacts with calcite. Calcite is a "salt" in that it dissolves in seawater forming ions (++Ca and 2 -HCO3). Freshwater is slightly acidic compared with seawater that is slightly "basic" so calcite dissolves easily in cold freshwater, and precipitates in seawater, assisted by biological activity.

A sinkhole is a cavity in the ground, typically in limestone bedrock, caused by water erosion and providing a route for surface water to disappear into cavernous passages underground. Sinkholes may form slowly over long periods of time, or they can form from the sudden collapse of the roof of a cavern such as when too much groundwater is pumped from a region where karst landscape features exist (as illustrated in Florida (Figures 12-52 and 12-53).
Guadalupe Peak - .Capitan Reef
Fig. 12-51.
El Capitan Reef is a massive exposure of limestone in Guadalupe National Park, Texas. Nearly 270 million years ago (in Late Permian time) this region was part of an large embayments of the sea that was surrounded my massive limestone reefs. The rock is now host to many caverns.
Round ponds are all flooded sinkholes formed by the collapse of caverns in the Orlando region of central Florida
Fig. 12-52. Central Florida is a region underlain by soft limestone deposited on a shallow Bahamas-like platform that existed several million years ago. When sea level fell, freshwater started dissolving underground passages in the limestone. When the caverns collapse they fill with water.
Florida sinkhole
Fig. 12-53.
Example of a recent sinkhole collapse in the Orlando region of central Florida. Sinkhole collapses can be rapid and destructive.
Caverns form mostly below the water table in regions underlain by limestone. Flowing water along fractures and faults erode and expand underground passageways. Over time, streams carving through limestone (karst) areas lower the stream base level, causing the water table to fall. This leaves passages that were once filled with water high and often quite dry. Once exposed to the air, water seeping into the cavern carries dissolved calcium carbonate. When the water evaporates, it leaves behind a residue of calcium carbonate called travertine. Travertine is a white or light-colored calcareous rock deposited from mineral springs; or a common name for freshwater limestone deposits. Most cavern speleothems consist of travertine. A speleothem is a structure formed in a cavern by the deposition of minerals (usually calcite) from water, including such features as stalactites, stalagmites, columns, flowstone, or other features found in caverns that form above the water table.

A stalactite is a tapering structure hanging like an icicle from the roof of a cave, formed of calcium salts deposited by dripping water. A stalagmite is a mound or tapering column rising from the floor of a cave, formed of calcium salts deposited by dripping water and often eventually uniting with a stalactite to form a column.

Website About Selected National Parks with Caverns

Guadalupe Mountains National Park
Carlsbad Caverns National Park
Great Basin National Park
Wind Cave National Park
Jewel Cave National Monument

Karst and Cavern Features
Fig. 12-54.
Landscape features associated with karst and cavern development.
Karst map of the United States
Fig. 12-55.
Regions where karst and caverns occur in the United States.
Fig. 12-56. Speleothems
: flowstone, stalactite, stalagmites, and columns
Mammoth Cave Main Entrance, Kentucky
Fig. 12-56
. The natural cavern entrance to Mammoth Cave, Kentucky is a massive sink hole.
Solution weathering in Mammoth Cave
Fig. 12-57.
Underground stream erosion carved this massive passage in Mammoth Cave, Kentucky.
The River Styx
Fig. 12-58.
The "River Styx" drains from an underground stream in the Mammoth Cave System.
Speleothems in Lehman Caverns, Nevada
Fig. 12-59.
Stalactite, stalagmites, columns, and travertine dams in Lehman Caverns, Nevada.
Flowstone in Carlsbad Caverns, New Mexico
Fig. 12-60.
Massive deposit of travertine "flowstone" in Carlsbad Caverns, New Mexico.
Massive stalagmites in Carlsbad Caverns, New Mexico
Fig. 12-61
. Massive stalagmites in a passage in Carlsbad Cavern, New Mexico.
Check out these websites!

The Water Cycle
Why is the ocean salty?(USGS)

Chapter 12 Quiz Questions