streams are dynamic systems that represent a balance between the forces that drive the current and those that resist the flow. Channels have many different styles that form in response to a quasi equilibrium between the gradient, or slope, of the streambed, the discharge of the stream, the amount of sediment being transported, the roughness of the streambed, and the resistance of the bank to erosion. The stream may form one of three main types of channels in response to the relative contributions of these variables. straight channels are the rarest and are usually controlled by incision into a bedrock structure, but within the straight channel the current usually follows a curved path. Meandering streams are most common, with the current actively eroding cut banks and depositing material on the opposite point bars. In this way the meanders move back and forth across the floodplain, maintaining equilibrium through changes in the sinuosity, meander wavelength, width and depth of the channel, and velocity of the current. Meandering channels and their floodplains are different parts of the same dynamic system. Braided streams have multiple channels and are prone to rapid changes; they carry more sediment than meandering or straight channels. They are prone to large fluctuations in discharge and load, and many are found in environments in front of melting glaciers.
Individual stream and river channels are parts of much larger systems, and the patterns of branching and angles between individual streams often define different patterns that reflect underlying processes. some river systems exhibit control by uniform slopes; some have rectilinear patterns reflecting underlying beds and structures; some are radial, reflecting drainage off domes; and others cut straight through uplifted mountain ranges. The regional pattern of the stream channels reflects control by the underlying geology, and the more local stream channel pattern reflects control by the balance between the forces driving the current and those opposing it. streams can deposit thick layers of sand, mud, and gravel on floodplains, cut through them forming terraces, and carry massive amounts of sediment to the sea to deposit them as giant delta complexes. These delta complexes build to sea and have forms that reflect a balance among sediment input, tides, and wave energy. Many delta lobes are active on the order of 1,000 years; then the river switches course and forms a new lobe, as the older one subsides.
See also drainage basin (drainage system); flood; river system; sedimentary rock, sedimentation.
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