Water On and Under the Ground 1
Mosi- oa-Tunya or “the smoke that thunders” is the largest single sheet of falling water. Today it is known as Victoria
Falls, part of the Zambezi River in southern Africa. It is used to power hydroelectric dams similar to the Niagara Power
Project in Canada.
The Hydrologic Cycle
Evaporation - The process by which water changes from liquid to vapor , powered by the sun
Transpiration – Water taken up by plants passes directly to the atmosphere through the leaves , powered by the sun
Condensation – The process by which water changes form a gas to ei ther a liquid or solid, dependent on pressure,
temperature and humidity.
Precipitation – The process by which condensed water in the atmosphere falls back to the surface
Surface runoff – Precipitation that drains over land or into stream channels
Infiltration – The process by which water seeps into the ground by working its way through small pores in soil.
Residence time – The average amount a time a water molecule spends in a reservoir
Water moves materials and energy between all four systems: the geosphere, hydrosphere, atmosphere and biosphere.
It is a constituent of many minerals such as micas and clays.
Because it is a closed system, the total amount of water is fixed, however all local reservoirs within the cycle such as lakes
and rivers are free to lose or gain water.
The vast majority of the Earth’s water is saline, frozen or underground. Visible fresh water such as lakes and rivers make
up less than one-hundredth of a percent of all water.
The largest reservoir by far is the world ocean, which holds 97.5% of the Earth’s water.
74% of the Earths fresh water is frozen in glaciers, and 98.5% of the fresh water that isn’t frozen is underground, and
1.5% is above ground.
Historically, hum an settlement has surrounded lakes and rivers. Now, however, underground water sources are much
There is a correlation between size or the reservoir and the residence time. In an ocean or ice sheet, average residence time
is several thousand years.
Water in the ground water system has a residence time of hundreds of years or more. Water will stay for weeks in streams
and rivers, for days in the atmosphere, and for hours in living organisms.
Although volumes of water are always transferri ng from one reservoir to another, in the short term, each reservoir volume
is approximately constant. However, in the long term the volumes may vary; for example: there was more frozen fresh
water during the last ice age.
How Water Affects Land
Stream – Water that flows downslope along a clearly defined passageway
Channel – The passageway through which a stream flows
Gradient – The steepness of a stream channel
Discharge – The amount of water passing through a stream’s bank per unit of time
Load – Suspended and dissolved sediment carried by a stream
Overland flow (sheet flow) – Water taking the form of a thin , broadsheet traveling down a slope
Stream flow – Concentrated water flow into well-defined passageways
Oxbow lake – Horseshoe-shaped previous watercourses of streams that have since been cut -off.
Floodplain – The relatively flat valley floor adjacent to a stream channel, where stream water overflows
Alluvium – Unconsolidated sediment, commonly triangular, that forms when a stream enters a standing body of water,
typical of semi-arid conditions where vegetation is sparse.
Delta - the built up sediment of an alluvium where a river meets a standing body of water.
Drainage basin – the total area from which water flows into a stream
Divide – a topographical high that separates adjacent drainage basins Water On and Under the Ground 2
Two processes (erosion and deposition) determine the shape of a channel. Gradient, discharge and load determine which
process is dominant at a specific location and time.
For example, if the gradient is steeper, velocity will increase. This causes a greater rate of discharge and load. As
a result, width and depth of stream will increase as a result of erosion.
Other factors that influence shape of a channel include:
• Topography which influences gradient
• Climate including precipitation which influences discharge
• Underlying rock and its resistance to erosion
Stream formations can be classified into three broad categories:
1. Straight Channel – relatively short stretches, high gradient, include a V-shaped valley as a result of mass wasting , near
the source. The deepest part of the channel oscillates from side to side.
Example: Alaskan mountain ranges
2. Meandering Channel – low gradient, form down stream close to the mouth. The erosion concentrates to the side of the
channel, rather than at the bottom. As water sweeps a round a bend, velocity increases around the outside of the curve,
undercutting and steepening it to form a cut bank. Along the inner bank, velocity is decreasing and sediment is
deposited forming a point bar. During period of heavy rainfall or flood, the stream may overflow its banks and the
water cuts across the most narrow part of the loop. The former meander is cut off and forms an oxbow lake.
Example: Phnom Penh, Cambodia
3. Braided Channel – arise when a stream’s ability to carry sediment varies over time, inconsistencies in flow and load
cause the river to divide and reunite over time. Found after snow melt or in streams with easily eroded banks. Usually
involves coarse sediment.
Example: Tasman River, New Zealand
Natural levees are created during over bank flooding when sand and silt are deposited next to a channel creating belts of
higher sand on either side. Deposition is heaviest closest to the channel so the levee slop es outward. Further away, finer
particles settle and create a floodplain.
Prominent deltas do not form where there are prominent currents. Delta only forms when the rate of deposition is greater
than the rate of coastal erosion.
Every stream is surrounded b y a drainage basin (sometimes called a catchment or a watershed). In general, the greater the
annual discharge of the river, the greater the drainage basin is.
Divides separate drainage basins. In North America, the Rocky Mountains and other features sepa rate the drainage basins.
The divide directs the flow of runoff. Thunder Bay is part of the Atlantic drainage basin.
Eutrophication – excessive algal growth due to agricultural fertilizers, causes lack of oxygen in water. Important in
Ephemoral lakes – short lived lakes, found in arid climates, often only intermittently filled with shallow water.
Water exits a lake by either evaporation or possible outlet. Salt lakes lack an outlet and so the water becomes more saline
over time, as water only leaves through ev