Chapter 9 Running Water and Groundwater Earth as A System The Hydrologic Cycle Hydrologic Cycle The unending circulation of Earth s water supply Infiltration When a portion of water from precipitation soaks into the ground Rate of rainfall Earths absorption rate Runoff Surplus water flows over the land into lakes and streams Transpiration When precipitated water infiltrates the ground and is absorbed by plants Evapotranspiration Combines the effects of evaporation and transpiration In very cold areas the precipitated water does not soak into the ground but rather it stays on land and become part of a snowfield or land Hydrologic cycle is balanced except for all the continents taken together in which precipitation exceeds evaporation The amount of water that is runoff depends on several factors Intensity Duration of rainfall Running Water 1 2 Amount of water in the soil 3 Nature of the surface material 4 Slope of the land 5 Extent and type of vegetation The runoffs then form tiny currents called Rills Which combine to form Gullies Which combine to form streams When the surface is highly impermeable runoff is dominant Drainage Basin o Land area that transports the runoff water into a river system o Divide An imaginary line that separates the drainage basin of one stream from another o Mississippi River has the largest drainage basin in North America River Systems o Includes the entire drainage system o Divided into 3 Zones 1 Sediment Production Located in the headwater region of the 2 Sediment Transportation Trunk streams transport the river system sediment 3 Sediment Deposition Sediment is deposited at the mouth of the river and forms a delta or is reconfigured by waves or moved off shore by ocean currents It is primarily fine sediments that reach the ocean o Even though one may be dominant sediment is always being eroded transported and deposited along the entire length of a stream Stream Flow Water can flow in one of two ways 1 Laminar Flow Slow moving streams and the water is roughly in a straight line that parallels the stream channel 2 Turbulent Flow Water moves in an erratic fashion characterized usually by a swirling motion Flow Velocity o Velocity increases as you move into deeper parts of the channel o Factors that influence flow velocities the streams ability to do work Erode 1 Gradient 2 Channel Size and Cross Sectional Shape 3 Channel Roughness 4 Amount of water flowing in the channel Gradient and Channel Characteristics o The slope of a stream channel expressed as a vertical drop of a stream over a specified distance o Stream with the higher gradient has the greater velocity Discharge o Measure most often used to compare the size of streams o The volume of water flowing past a certain point in a given unit of o Determined by multiplying a stream s cross sectional area and time velocity o As discharge increases the width depth and flow of velocity increases predictably Changes From Upstream to Downstream o Longitudinal Profile Cross Sectional view of a stream from its headwater to its mouth Most obvious feature is its concave shape Result of the decrease in slope that occurs from the headwater to the mouth o Increases in channel size and discharge and decreases in channel roughness that occur downstream because its eroded away compensate for the decrease in slope downstream The Work of Running Water Stream Erosion o Raindrops can help erode by knocking sediment particles loose o Streams erode unconsolidated material and cuts a channel into o Streams ability to erode bedrock is enhanced by the particles it solid bedrock carries Transportation of Sediment o Streams transport sediment in 3 ways In Solution Dissolved Load In Suspension Suspended Load 1 2 3 Sliding Or Rolling Along the Bottom Bed Load o Dissolved Load Brought to a stream by groundwater and is dispersed throughout the flow Velocity of a stream flow has essentially no effect on a stream s ability o Suspended Load Only fine particles are carried in this way Type and amount of material carried in suspension are controlled by two factors 1 Flow Velocity 2 Settling Velocity Speed at which a particle falls through a still fluid o Bed Load Coarser materials move along the bottom bed of the stream and constitutes the bed load Grinding action of the bed load is of great importance Saltation Sediment appears to jump or skip along the stream bed Increase in velocity Suspension Decrease in velocity Bed Load o Competence and Capacity Streams ability to carry solid particles is described using two criteria 1 Capacity The maximum load of solid particles a stream can transport unit of time 2 Competence Measure of a streams ability to transport particles based on size rather than quantity Flow Velocity is key Deposition of Sediment o When stream flow drops below the setting velocity of a certain particle size sediment in that category begins to settle out Sorting Particles of similar size are deposited together o Alluvium Any stream deposited sediment Stream Channels Can be divided into two types 1 Bedrock Channels Streams actively cut into solid rock 2 Alluvial Channels Beds and banks are composed of unconsolidated sediment Bedrock Channels o Headwaters are where the gradient is usually steeper o Alternates between relatively gently sloping segments alluvial deposition and steeper segments where bedrock is exposed Alluvial Channels o Alluvium sediment are loosely consolidated more erosion more shape changing o Two common types of Alluvial Channels 1 Meandering Streams Streams that transport much of their load in suspension Most erosion is focused on the outside where turbulence is greater deposition of sediments on the inside without shape change Because the outside is where the erosion takes place it is called a Cut Bank Coarser material is deposited as Point Bars Cut Off Shorter channel segment Oxbow Lake The abandoned bend that is formed when a river erodes through the narrow neck of the land When channels have an interwoven appearance When flow is very slow this causes coarser materials to deposit and splits the flow into different paths 2 Braided Streams Base Level and Stream Erosion Shaping Stream Valleys Base Level Lowest elevation to which a stream can erode its channel Two types of base levels The level at which the mouth of the stream enters the ocean 1 Ultimate Base Level The sea level 2 Temporary Local Base Levels Lakes resistant layers of rock and mainstreams that act as base levels for their tributaries
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