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The drainage divide represents the boundary between adjacent drainage basins and determines into which basin precipitation flows. Streams within the drainage basin are either perennial or intermittent in flow. Perennial streams flow all year. The base flow of these streams is provided by groundwater seepage into the channel. Tributary streams are small streams that enter into the main stream. Tributary streams, especially the smaller ones around the periphery of the basin, are intermittent. Intermittent streams only flow during wetter times of the year. Much of their flow is provided by surface runoff and when the water table is higher as a result of moist conditions. The upland between tributaries is called an interfluve.
A stream that originates in a moist region that flows into and through a drier region. The Nile River is considered an exotic stream.
The Davisian theory begins with the uplift of land or fall of base level, initiating a cycle of erosion from youth characterized by downcutting, to mature (valley widening), and finally old age stage (formation of a peneplain). The cycle is renewed with uplift or a fall in base level.
The dynamic equilibrium theory describe stream development as constantly adjusting to inputs (water) and outputs (sediment)s to the stream system. Over time, the channel achieves an equilibrium and remains in this state until a disturbance alters the inputs and outputs within the stream system.
A dendritic drainage pattern is the most common form and looks like the branching pattern of tree roots. It develops in regions underlain by homogeneous material.
A parallel drainage pattern forms where there is a pronounced slope to the surface. A parallel drainage pattern also develops in regions of parallel, elongate landforms like outcropping resistant rock bands. Tributary streams tend to stretch out in a parallel-like fashion following the slope of the surface.
A trellis drainage pattern looks similar to their namesake, the common garden trellis. Trellis drainage develops in folded topography like that found in the Appalachian Mountains of North America.
The rectangular drainage pattern is found in regions that have undergone faulting. Faulting off-sets the direction of the stream with tributary streams making sharp bends and entering the main stream at high angles.
The radial drainage pattern develops around a central elevated point. This pattern is common to such conically shaped features as volcanoes.
The centripetal drainage pattern is typical of basins in arid regions with interior drainage. During wetter portions of the year streams feed ephemeral lakes, which evaporate away during dry periods leaving salt flats.
A deranged (also know as contorted) pattern develops from the disruption of a pre-existing drainage pattern. See: Drainage Patterns
Antecedent streams flowed across bedrock structures prior to uplift. Slow mountain building permitted stream erosion to keep pace with uplift. The Columbia River that cuts across the Cascade Mountains is an example of an antecedent stream. Superimposed streams expose buried geologic structures and cut through them.
The cross-sectional area of the stream is determined by multiplying channel depth by channel width along a transverse section of the stream.
Channel slope is the difference in elevation between two points on a stream divided by the distance between them measured along the stream channel.
The wetted perimeter is the portion of the channel that is "wet". The wetted perimeter is the width plus twice the depth that the water touches.
Generally, the larger the discharge, the smoother the channel, greater the stream velocity. Cross-sectional area and discharge increases down stream due to tributary and ground water flow into the channel.
As streams grow larger their down stream slope decreases, preventing a continuous buildup of energy and creating a more uniform distribution of stream energy along its length.
The sinuosity ratio is used to determine whether a channel is straight or meandering. It is calculated as the distance between two points on the stream measured along the channel divided by the straight line distance between the two points. If the sinuosity ratio is 1.5 or greater the channel is considered to be a meandering one.
Straight channel exhibit a linear though rarely is a channel perfectly straight in nature. A meandering channel is one that takes twists and turns over its length. Abraided channel is one in which the main channel is separated into several smaller channels by bars.
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