Cliffs or escarpments and differences in the resistance of rocks create irregularities in the bed of a river and can thus cause rapids and waterfalls . A river tends to eliminate irregularities and to form a smooth gradient from its source to its base level. As it approaches base level, downward cutting is replaced by lateral cutting, and the river widens its bed and valley and develops a sinuous course that forms exaggerated loops and bends called meanders. A river may open up a new channel across the arc of a meander, thereby cutting off the arc and creating an oxbow lake. 2 Rivers modify topography by deposition as well as by erosion.
River velocity determines quantity and size of rock fragments and sediment carried by the river.
When the velocity is checked by changes of flow or of gradient, by meeting the water mass of lakes or oceans, or by the spreading of water when a stream overflows its banks, part of the load carried by the stream is deposited in the riverbed or beyond the channel. Landforms produced by deposition include the delta , the floodplain , the channel bar, and the alluvial fan and cone. The discharge, or rate of outflow, of a river depends on the width of its channel and on its velocity. River volume is affected by duration and rate of precipitation in the drainage basin of the river. A river system may be enlarged by piracy, or the process by which one river, cutting through the divide that separates its drainage basin from that of another river, diverts the waters of the other into its own channel.