RIVER TERRACE

bench or step that extends along the side of a valley and represents a former level of the valley floor. A terrace results from any hydrological or climatic shift that causes renewed downcutting. It generally has a flat top made up of sedimentary deposits and a steep fore edge, and it may be the remains of an old floodplain, cut through by the river and left standing above the present floodplain level. Another type of terrace is cut into bedrock and may have a thin veneer of alluvium, or sedimentary deposits. In paired terraces, the terrace features on each side of a valley correspond. The river system through time Natural river systems can be assumed to have operated throughout the period of geologic record, ever since continental masses first received sufficient precipitation to sustain external surface runoff. The Precambrian portion of the record, prior to 570,000,000 years ago, is complicated by the widely metamorphosed character of the surviving rocks, although even here the typical cross-bedding of shallow-water sands can be recognized in many places. The Cambrian and post-Cambrian succession of the last 570,000,000 years contains multiple instances of deposition of deltaic sandstones, which record intermittent deposition by rivers in many areas at many intervals of past time. The span since the Precambrian is long enough, at present rates of erosion, for rivers to have shifted the equivalent of 25 to 30 times the bulk of the existing continental masses, but the rate of erosion and sedimentation is estimated to have increased with time. Of necessity, river systems now in existence date from times not earlier than the latest emergence of their basins above sea level, but this limitation allows numbers of them to have histories of 100,000,000 years or more in length. Drainage diversion by stream capture A river system of appreciable size is likely to have undergone considerable changes in drainage area, network pattern, and profile and channel geometry. Adjoining streams compete with one another for territory. Although competition is effectively nil where divides consist of expanses of plateau or where opposing low-order streams of similar slope flow down the sides of ridges, it frequently happens that fluvial erosion is shifting a divide away from some more powerful trunk stream and toward a weaker competing trunk. In extreme cases, the height difference is so marked that a tributary head from one system can invade, and divert, a channel in the adjoining system: such diversion, termed stream capture, has already been noted as a principal mechanism in the adjustment of network patterns to structural patterns. Close general adjustment to structure implies multiple individual adjustments, unless the stream network has developed solely by the headward extension of tributaries along lines of structural and lithologic weakness: the network predicated on a single regional slope is dendritic in pattern. By encroachment and capture a successfully competing stream becomes yet more powerful, the headward extension of its basin increasing the discharge of the trunk channel and permitting reduction of slope; i.e., additional downcutting. Seaward extensions of basins occur where deltas lead the outbuilding of alluvial plains and where crustal uplift (and also at times strandline movements) result in emergence. Conversely, basin area is reduced along the seaward edge by submergence, in response to crustal depression or rise in sea level. The potential limits to basin size are fixed by available areas of continent with surface moisture surplus, in combination with theoretical optimum shape of basin; however, actual basin shapes, for all large rivers, are to some extent affected by crustal deformation.