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The hydrologic cycle, also known as the water cycle, summarizes the circulation of water on the earth's surface.
Running water can exhibit two general types of flow: laminar flow occurs if there is no mixing and individual streamlines lie parallel to one another, while turbulent flow occurs when streamlines become intertwined. Turbulent flow is, of course, the most common type of flow observed in stream channels.
A number of aspects of river channels can be described:
The discharge of a river (the total volume of water moving past a point during a given period of time) can be calculated by multiplying the width, depth and velocity together.
Streams erode and transport sediment. As the loose sediments is moved along the bottom of the river channel, small bedforms can develop, such as ripples and sand dunes.
The total load (quantity of sediment) of a stream can be described as consisting of three components:
The maximum size of particle that is stream is capable of transporting is termed its competence, and is strongly related to velocity. The capacity of a stream is a measure of the total load that a stream can carry, and is controlled by the discharge.
All material moved and deposited by streams is termed alluvium. Alluvial rivers are those streams whose bed and banks consist of alluvium, and can be divided into three main types: braided, meandering and straight channels.
Deltas are depositional landforms that are composed largely of alluvium, where a river enters a standing body of water and deposits its load. Alluvial fans are similar, but are deposited by streams in areas where there are abrupt changes in channel gradient. These landforms are best observed in arid settings at the base of a mountain range.
The lowest point to which a stream can erode its bed is termed base level. For most streams, this is sea level.
The floodplain of a stream is the low lying land adjacent to the channel that periodically is flooded. The position of an alluvial stream channel can, through time, move across the floodplain as the channel erodes and redeposits the sediment.
A drainage basin is the area of land that drains to a given stream, and is bound by a high topographic ridge termed a drainage divide. The drainage pattern is the arrangement of streams within a drainage basin. For example, the most common drainage pattern is termed dendritic, because streams tend to branch into smaller and smaller tributaries in an upstream direction.
A key concept in understanding river behavior is that they behave as systems - that is, disturb one part of the stream channel and the impact will move both in an upstream and downstream direction. For example, the construction of a dam will cause deposition in the reservoir and erosion downstream from the structure.
About one-quarter of the earth's fresh water is found as groundwater.
Another look at the water cycle shows that some water can percolate underground, and accumulate in aquifers as groundwater. As an introduction to this topic, here is a short article that Dr. Buchanan wrote about the Spokane aquifer for The Inlander, a local newspaper.
Even in "solid" rock, water can move underground through cracks and pores. Porosity is the measure of how much void space exists in a rock, while the permeability is a measure of the ability of the rock to transmit water. If geologic material can store and transmit groundwater in economic quantities, it is termed an aquifer.
Aquifers can generally be categorized as either confined or unconfined.
Unconfined aquifers are also referred to as water table aquifers. There is a clear zone of aeration between the ground surface and the water table (where pores are filled with both air and water), below which there is the zone of saturation (where all the pores are filled with water). These aquifers are usually recharged (replenished) directly from the surface, and as such, are prone to contamination by various land use activities. Groundwater will move in the direction of the slope of the groundwater table.
In confined aquifers, also known as artesian aquifers, there are confining beds above and below the water-bearing zone. These confining beds (known as aquicludes) have greatly reduced permeability and so they serve to "confine" the groundwater. Due to the confined nature of these aquifer systems, contamination of the groundwater from surface activities is usually not a significant problem. However, these aquifers tend to be limited in their recharge. Groundwater movement in these aquifers is largely governed by the pressure in the system.
Springs are natural points of groundwater discharge on the surface of the earth. Hot springs and geysers are special cases where circulating groundwater comes in close proximity to hot rock or magma at depth.
As groundwater circulates through soluble rocks, like limestone, large voids or caves can form due to the solution process. Such areas are termed "karst topography" and are characterized by the presence of caves, sinkholes, sinking streams and blind valleys. Mammoth Cave National Park in Kentucky is a particularly good example of a karst region in the central U.S.
A number of environmental problems are associated with groundwater use: declines in the water table elevation on a regional scale (groundwater mining), land subsidence due to collapse of pore spaces when groundwater is withdrawn, groundwater contamination by landfills, spills and septic systems, and salt water incursion along coastal zones. Also, "interference" may result between two water wells located close to one another when they affect each other during pumping. That is, when one well pumps and the cone of depression expands around the well, the water level in the neighboring well may be lowered, and in some cases, dry up completely (call the attorneys!).
The transfer of rock and soil materials downslope under the influence of gravity is technically termed "mass wasting."
Some of the factors that influence slope stability are:
Geologists classify mass movements based on their rate of motion, the nature of the material, and the type of motion involved.
For example:
Once a mass movement has occurred, is there any way to reduce or mitigate the threat? Yep - engineers can design retaining walls, use rock bolts, install drain pipes, and reduce the slope angle in areas prone to landsliding.
Follow this link to more information about the landslides that occurred in Washington state in February 1996.
Seismology is the study of earthquakes, which are vibrations in the Earth in response to a rapid release of energy.
Strain energy is released at a point in the Earth's crust termed the focus, with the epicenter located on the surface of the Earth immediately above the focus. Seismic waves are generated at the focus, and radiate outward in all directions. Several types of seismic waves are generated during an earthquake event: P waves, S waves and surface waves.
Earthquakes are measured by devices called seismographs, which make recordings of earthquake events termed seismograms. The Southern Arizona Seismic Observatory has an on-line seismometer that is stationed in the Tucson, Arizona area. By recognizing the arrivals of the P waves and S waves at a seismic station one can determine the distance to the focus. Combining the data from at least three stations (by triangulation) will reveal the approximate location of the focus and epicenter.
The strength of an earthquake can be reported by either the Richter scale (earthquake magnitude) or the lesser known Mercalli scale (earthquake intensity).
The distribution of earthquakes in the Earth's crust closely follows plate boundaries. Here is a map view of recent earthquakes along the western U.S.
Follow this link to more information about the occurrence of earthquakes in the Pacific Northwest region.
