How do stream valleys and their channels and floodplains develop? As a stream flows, it carves a valley and creates a floodplain on either side of its channel. The valley may have steep to gently sloping walls. The channel may be straight, meandering, or braided. During normal, nonflood periods, the channel carries the flow of water and sediments. During floods, the sediment-laden water overflows the banks of the channel and inundates the floodplain. The velocity of the floodwater decreases as it spreads over the floodplain. The water drops sediments, which build up natural levees and floodplain deposits.
How do drainage networks work as collection systems and deltas as distribution systems for water and sediment? A stream and its tributaries constitute an upstream-branching drainage network that collects water and sediments from a drainage basin. Each drainage basin is separated from other drainage basins by a divide. Drainage networks show various branching patterns—dendritic, rectangular, trellis, or radial. Where a river enters a lake or ocean, it may drop its sediments to form a delta. At the delta, the river tends to branch downstream to form distributaries, which drop the river’s sediment load in topset, foreset, and bottomset beds. Deltas are modified or absent where waves, tides, and shoreline currents are strong. Plate tectonic processes influence delta formation by providing uplift in the drainage basin and subsidence in the delta region.
How does flowing water in streams erode solid rock and transport and deposit sediment? Any fluid can move in either laminar or turbulent flow, depending on its velocity, viscosity, and flow geometry. The flows of natural streams are almost always turbulent. These flows are responsible for transporting sediment in suspension, by rolling and sliding along the bed, and by saltation. The settling velocity measures the speed with which suspended particles settle to the streambed. Running water erodes solid rock by abrasion; by chemical weathering; by physical weathering as sand, pebbles, and boulders crash against rock; and by the plucking and undercutting actions of currents. When a current transports sand grains by saltation, cross-bedded dunes and ripples may form on the streambed.
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How does a stream’s longitudinal profile represent an equilibrium between erosion and sedimentation? A stream is in dynamic equilibrium when erosion balances sedimentation over its entire length. Topography, climate, discharge and velocity, resistance to erosion, and sediment load affect this equilibrium. A stream’s longitudinal profile is a plot of the stream’s elevation from its headwaters to its base level.