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Chapter 6

GLG110 Textbook Notes Exploring Geology 2nd Ed Chapter 6

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Earth Sciences

[GLG Text Notes Chapter 6] 1 Chapter 6: Running Water The hydrologic cycle Running water is dominant agent of landscape alteration, eroding more terrain and transporting more sediment than any other process Because so many people live near rivers, floods are among most destructive of all geo hazards Water: 97.2% stored in global oceans, 2.15% in ice sheets and glaciers, and 0.65% divided among lakes. Streams, subsurface water, and the atmosphere Hydrologic cycle: unending circulation of Earths water supply; energy provided by Sun then water evaporates into atmosphere from oceans and to a much lesser extent, continents, then winds transport this moisture laden-air, often for great distances, until conditions cause the moisture to condense into clouds and precipitation to fall and so on Water that hit land soaks into ground, called infiltration, moving downward, then laterally, and finally seeping into lakes and streams, or directly into the ocean When rate of rainfall greater than the lands ability to absorb it, additional water flows over the surface into lakes and streams, process called runoff Some of water also get absorbed by plants and later released into atmosphere, called transpiration Because one cannot clearly distinguish between the amount of water that is evaporated and the amount that is transpired by plants, the term evapotranspiration is used to combine the effect o When precipitation falls in very cold areasat high elevations or high latitudesthe water may not immediately soak in, run off, or evaporate; it may instead become part of a snowfield or glacier Important that hydrologic cycle stays balanced; since the total water vapour in the atmosphere remains about the same, average annual precipitation over Earth must be equal to the quantity of water evaporated; but this is not true since evaporation exceeds precipitation Running water is the single most important agent sculpting the Earths land surface Runoff initially flows in broad, thin sheets across the ground, appropriately termed sheet flow [GLG Text Notes Chapter 6] 2 The amount of water that runs off in this manner rather than sinking into the ground depends on the infiltration capacity of the soil o Infiltration capacity is controlled by: intensity of rainfall, prior wetted condition of the soil, soil texture, slope of land, nature of vegetative cover After flowing as a thin, unconfined sheet for a short distance, threads of current typically develop, forming tiny channels called rills To be considered a true stream, a body of water must be confined to one or more trough-like linear depressions, called channels, and to flow along a definite course Basic components of a stream Between its source area, called the head or headwaters, and its mouth where it empties into another water body, a stream follows a well-defined path or course; any given segment of a stream along its course is a reach Especially in low-lying areas, stream channels commonly develop snake-like bends or loops called meanders The bottom, or bed, of a stream channel is bounded on either side by banks, which confine the flowing water during times of normal flow Floodplains are areas that are submerged in water only during floods and are flatter than the normal surrounding area If we were to examine the longitudinal profile (side view) of a major stream we would notice that the slope or gradient of its channel is far from constant (it is concave up) Characteristics and controls of stream flow Water can flow two ways: laminar flow (water particles flow in straight paths that are parallel to the channel) or turbulent flow (water moves in a chaotic and erratic fashion that is commonly characterized by swirling, whirlpool-like eddies); streams velocity determines which flow type exists The cross-sectional shape of a channel determines the amount of water in contact with the channel and hence affects the frictional drag; the most efficient one is the one with the least perimeter The size and roughness of the channel also affect amount of friction [GLG Text Notes Chapter 6] 3 Rapids may form where turbulent flows extends all the way up to the water surface because of the violent collision and diversion of flowing water around irregularities on the stream bed Discharge of a stream is the amount of water flowing past a certain point in a given unit of time; discharge (m /s) = channel width (m) x channel depth (m) x velocity (m/s) Changes from upstream to downstream A stream can maintain higher velocity near its mouth, where it has a lower gradient, than upstream because of the greater discharge, larger channel, and smoother bed Stream erosion Streams erode their channels mainly by the plucking of loosened particles from their beds and by abrasion; some erosion can also result from the dissolution of soluble bedrock and channel debris; most dissolved material in a stream is contributed by inflows of groundwater When an eddy is strong enough it can dislodge particles from the channel and lift them to the moving water thus causing swift erosion Common features on some riverbeds are rounded depressions known as potholes; these are created by the abrasive action of particles swirling in fast- moving eddies which causes a rotational movement like a drill Transport of sediment by streams Streams transport their loads of sediment in 3 ways: o Dissolved load: in solution o Suspended load: in suspension o Bed load: along the bottom of the channel Settling velocity: is defined as the speed at which a particle falls through a stationary fluid The particles that make up the bed load move along the bottom by rolling, sliding, and saltation (leap along) The max load of solid particles that a stream can transport is terms its capacity
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