Earth Science
Surface Water & Groundwater

The Planet "Aqua"

    Were an alien intelligence to visit our solar system, they could conceivably call our planet "Aqua" (or "Water") due to Earth's striking presence of abundant liquid water. In fact, about 72% of Earth's surface is covered by the oceans, with freshwater making up only about 1% of the total supply by volume. Among the planets in the solar system, only Earth has an abundance of liquid water at the surface, because of its fortuitous orbital distance from the Sun. (The planet Uranus and some of the moons of Jupiter, such as Callisto, are thought to have abundant liquid water, but this water exists below a frozen surface.)

    The term hydrosphere describes all of the earth's oceans, surface water, and groundwater, as well as the polar ice caps and atmospheric water combined.

    Surface water is a term usually applied to the collective freshwater on land, that first falls as precipitation (more or less pure freshwater) and collects in rivers, streams, ponds, lakes, as well as on the surface of the land.  Surface water moves down slope under the influence of gravity, and creates streams, rivers, lakes, and associated land forms over geologic time, and eventually reaches what is called base level (usually sea level), where sediments carried by the streams are usually deposited.

    Surface water is a key agent of weathering and erosion.  Weathering is a term that describes both physical and chemical changes in the minerals and rocks caused by differences in the environment in terms of pressure and temperature.  Physical (mechanical) weathering produces smaller and rounder sediment particles over geologic time.  Chemical weathering produces changes in the chemistry of the constituent minerals in a rock, towards a more stable composition for that particular environment.  Erosion describes the natural process that wear away rocks, as agents such as water, wind, and ice pick up and transport sediment from a source area to a basin, or area of deposition.

Stream Processes

    Streams are natural channels that convey water downslope by gravity.  Although streams vary in size and location, all streams perform three basic geological processes: erosion, transport, and deposition of sediments.  Streams typically originate in hilly or mountainous areas as meltwater from melting ice and snow, beginning as small gullies.  As these gullies flow, they erode the surrounding rocks and soils into a channel.  Broken bits of surrounding rocks are carried in the stream channel as sediment.

Sediment Transport

    Over geologic time, streams move enormous amounts of sediments, making water the most efficient agent of weathering and erosion.  Through stream erosion, entire mountain ranges can be worn down eventually to a flat peneplain, given enough geologic time.  Some of the most dramatic examples of stream erosion include the Grand Canyon, Mammoth Cave, Niagara Falls, and the Three Gorges of China.  The Mississippi River drains 42% of the U.S. waterways.

    The volume of water flowing through a stream during a unit of time is known as its discharge.  In general, the greater the discharge, the greater the volume of sediment moved.   Note that the discharge of a wide channel slowly flowing over a gentle slope is normally exceeded by the discharge of a narrow channel flowing quickly down a steep slope.

   The behavior of sediment particles within a stream is influenced by shape, size and composition.  Sediment particles normally reside on the bottom of the stream bed, and only move if water flows fast enough to dislodge them from the resting position.  In general, the larger the sediment particle, the heavier it is; and therefore, the faster water must flow in order to be able move the sediment.  Rounder particles move more efficiently than flat or irregular shaped sediments.

    The movement of large, heavy, coarse-grained sediments is referred to as bed load, and is characterized by a combination of irregular movements known as hydraulic action, in which sediment transport is impeded by friction with the stream bed, including: rolling, bouncing, sliding, traction load, and continuous contact.    Sometimes, the stream velocity is only sufficient to make heavier sediment particles "jump" or "skip" along the stream bed.  This type of sediment transport is called saltation.   Turbulence caused by rapidly moving water and partially suspended or churned up sediments can create a scouring action, known as abrasion, can sometimes dislodge other sediment particles and cause them to be transported by suspension and/or hydraulic action.  The total effect of abrasion on sediment transport can often exceed that of hydraulic action.

    The most effective way to transport sediment is suspension, if the stream velocity is fast enough to lift the particles off the stream bed.  The larger the diameter of the sediment, the heavier the sediment is, and the faster water must move in order to suspend these sediments.  Consequently, fine sediments such as mud (clay-sized), silt, and sand are transported much farther than coarse sediment such as boulders, cobbles, and gravel.  The term for the collective amount of fine (silt and clay) sediments carried by the stream in suspension is referred to as the suspended load.

    Because water is a chemically active substance, a small percentage of the minerals (i.e. calcite from limestone, and silica from sandstone) of the bedrock over which the stream flows is dissolved through chemical weathering, through a process called dissolution.  The collective amount of dissolved minerals is referred to as the dissolved load.

Sediment Deposition

    When stream velocity slows down (usually due to a reduction in the angle of slope, or the merging of a stream with a larger body of water such as a lake or ocean), sediments fall out of suspension and settle to the bottom, in a process called deposition.  The fate of all sediments is to eventually be moved from a source area to a basin, where sediments accumulate by deposition.  Sediments eroded from a mountain on land are carried by a stream down to it mouth at the ocean where they are deposited.  However, some of these off-shore sediments may be carried further down to the deep ocean floor by underwater mass movements such as landslides.  In some areas of the world, where the tectonic plates collide, there are unusually deep troughs known as deep ocean trenches which may reach depths of over 35,000 feet below sea level.  Sediments that fall into these trenches may eventually be dragged into the earth's interior, where they may melt and become part of a future magma.

    One of the features of sediment deposition is the creation of bars (as in "sand bars"), and stream meanders (in which the stream channel zig-zags over a relatively flat terrain).  If you visualize a cross-section of a stream, you should note that water in the center of the channel flows more quickly than water at the bottom or side of the channel, which is slowed down by frictional drag.

Young Stream: Deep, V-Shaped Channel
    In a hilly terrain, a stream flows downhill quickly, most of the erosion is "down-cutting," that is, the erosion is concentrated at the bottom of the stream bed, creating a relatively narrow, deep, V-shaped channel which is mostly straight and free from extensive meanders or floodplain.  This stage of stream evolution is referred to a a geologically young stream.

Mature & Old Age Stream: Wide, U-Shaped Channel
    When a stream slows down in an area where the slope is general, the water begins to erode the side banks rather than the stream bed, creating a wide, U-shaped channel, and a meandering stream course. Such a stream, called a geologically mature stream; creating point bars on the slower moving "inner bend" of the stream, while eroding cut banks on the faster flowing "outer bend" of the stream.  Stream meanders may change over time, and sometimes may be abandoned when the river cuts a straighter "short-cut," leaving a crescent-shaped, abandoned river channel known as an oxbow lake.

    An old age stream is marked by extensive meanders; a very wide, shallow channel; an abundance of fine-grained sediment due to a very low angle of slope (nearly level); and an extensively developed floodplain.  Within the floodplain, there may be tributaries that may hold water only during flood stage, known as yazoo tributaries.  The Missisippi River near its mouth by the Gulf of Mexico is a good example of an old age stream.

Oxbow lake formation

    Slow stream velocity causes sediment deposition to occur; combined with the shallower depth of the channel, a mature stream is more prone to flooding, in which water may flow over the river banks, carrying sediment off to the land adjacent to the stream channel. Over geologic time, the sediments carried by periodic flooding ("overbanked discharge"), on the average, every 2.5 years, produces a floodplain.  The mounds of sediment caused by flooding are called natural levees.  Floodplains have historically been heavily settled by people because they are productive soils which promote agriculture. In some cases (such as the Yellow River in China) due to progressive sediment accumulation, the stream bed may actually be higher than the surrounding floodplain, making it necessary to reinforce the natural levees to mitigate severe flooding. Floodplains are an integral part of the river where excess discharge is stored.  People who inhabit floodplains should therefore not be surprised that flooding occurs periodically.

The Watershed
    The watershed is a concept that treats all surface water in terms of a single integrated unit within boundaries determined by the topography (shape) of the land.  The boundaries of a watershed are the highest points (usually the crests of hills, called the watershed divide) defining the perimeter of the watershed.


    Groundwater is the collective water that exists in porous rock formations and/or unconsolidated sediments, such that this water fills in the void spaces in what is known as the saturated zone.  The top surface of the groundwater is called the water table or piezometric surface of the saturated zone.  Above the water table, the rock formations through which rain water may percolate downwards to recharge the water table is known as the unsaturated zone, or the zone of aeration.   There is collectively more groundwater than all surface water sources combined.  In land-locked areas, groundwater is the main source of freshwater and is usually extracted by drilling wells.

    Note in the diagram above that groundwater  is hydraulically connected with surface water; allowing streams to flow even during periods of extended drought.  The top surface of the groundwater, called the water table, may fluctuate depending upon topography (it rises and falls with the terrain), recharge (heavy rainfall may temporarily elevate the water table) or drawdown (lowering of the water table may occur through groundwater withdrawal via well pumping).

    Groundwater does not move in the way that surface water does because the path is not smooth - groundwater must wind a very slow, twisting path between void spaces in porous rock or unconsolidated sediment.  However, groundwater may not always be retrieved from water-bearing rocks or sediments; it is not enough for a rock to contain void spaces (porous); the pore spaces must be interconnected.

    Rock formations containing extractable ground water are known as aquifersRocks that are rich in clay minerals (such as shales) "bind up" water through adsorption, such that the water is not extractable, and are classified as aquitards.  An extreme example of an aquitard is called an aquiclude.  Aquicludes are often used to locate hazardous waste landfills because they provide a natural barrier to leaking waste containers.

    The rate of groundwater flow through a rock formation is called the permeability. Aquifers have relatively high permeability, which aquitards have low permeability.  Aquicludes have extremely low permeability.

    Sedimentary rocks, soils, glacial till, and fractured bedrock (such as limestone) have fairly high permeability, and generally make the best aquifers.  Clay-rich rocks (shales) or unfractured rock, such as igneous or metamorphic rocks, have low permeability, and are generally poor sources of groundwater, and would be generally considered aquitards.  Aquicludes may act as a confining layer preventing the downward movement of groundwater, and sometimes creating a locally elevated or perched water table.  If the perched water table outcrops at the surface (note in the diagram below  the left edge of the perched water table ends at the slope of the hill), groundwater may form a spring or a seep.

    If the groundwater recharge area is elevated (because the aquifer is exposed in a hilly or mountainous area), and there are also confining layers of rock above and below the aquifer, an artesian aquifer may be produced.  If the recharge area is at a higher elevation and the well drilled into the artesian aquifer, this well may flow by itself without pumping, because gravity supplies the energy to make this a flowing artesian well, sometimes called a spring.

©2002 by William K. Tong