Rivers

Rivers and streams are bodies of flowing surface water that transport sediment from continental highlands to lakes, alluvial fans, and ultimately the ocean. Streams are the main agent of erosion of the earth's continental crust, and they play a major role in shaping the landscape. Streams are also a focus of humans' interaction with our environment. Human agriculture, industry, and essential biology require fresh, accessible water. Ancient human civilizations first arose in the fertile valleys

The Nile River as viewed from the space shuttle. The Nile is the longest river in the world. AP/Wide World. Reproduced by permission.

of some of the world's greatest rivers: the Yangtze and Yellow Rivers in China, the Tigris and Euphrates Rivers in the Middle East, and the Nile in Egypt. The distribution of the earth's river systems has influenced human population patterns, commerce, and conquest since then, and the availability of uncontaminated surface water for irrigation, industrial and municipal uses remains a pressing geopolitical issue.

Streamflow is a gravity-driven process that acts to level continental topography. Stream erosion balances uplift at plate tectonic boundaries by mechanically and chemically eroding upland rocks, and transporting the resulting siliclastic sediments and dissolved ions and molecules toward the ocean. Current velocity determines a stream's capacity to transport a given volume of suspended and bedload sediment. Sediment transport is intermittent, and individual grains are deposited and re-entrained by turbulent streamflow many times before final deposition in deltas and alluvial fans.

Stream erosion and deposition act in dynamic equilibrium to maintain a concave longitudinal stream profile, called a graded profile, with steep headwaters to low-gradient downstream portions. The elevation where a stream enters another body of water, called base level, controls the downstream end of a stream profile, and the elevation of the headwaters determines the upstream end. Streams cannot erode below base level. Sea level is the ultimate base level for most river systems, and a sea-level change creates a string of compensatory adjustments throughout a stream system. Base level for an individual tributary, however, is controlled by the elevation of the next body of water it enters. If base level falls, or uplift occurs, current velocity increases, and the stream erodes downward. If base level rises, or subsidence occurs, a stream slows down and deposits sediment.

Streams flow in valleys that encompass an area between uplands. Some rivers carve their own valleys, and some flow in preexisting valleys created by other geologic processes like rifting or glacial erosion. The stream channel that contains flow during non-flood times runs through the stream valley flanked by its overspill areas called floodplains. Over time, a stream fills its valley with its own deposits; the stratigraphy of a river basin thus shows the depositional history of the stream. Most streams have a valley, a channel, and a floodplain, but their morphology varies between three end-member typestraight, meandering, and braidedepending on the stream gradient, the rate of sediment supply, and the sediment grain size.

Straight streams develop in regions where uplift and/or base level fall force rapid regrading by channel incision. Meandering streams develop at the low-gradient, downstream ends of stream profiles. Because they cannot erode below base level, streams near base level maintain their profile by moving horizontally across the stream valley, eroding and depositing sediment with little effect on the overall sediment flux. Meandering streams develop an organized pattern of fluvial landforms and deposits: coarse-grained point bars, gravel channel lags, sandy natural levees, abandoned meanders called oxbow lakes, and fine-grained flood deposits. Braided streams form in mountainous and glaciated areas where rapid currents, voluminous sediment supply, and coarse-grained sediment prevent a stream from forming an orderly pattern of channels and bars. Braided streams have many interlaced channels separated by longitudinal gravel bars that shift over time.

Stream systems are organized into drainage basins with small tributary streams that feed into larger trunk streams, and finally into a major river that lets out into the ocean. Drainage divides are topographic highs that separate drainage basins. Drainage basins and divides vary in scale from small hillside watersheds separated by ridges, to the two halves of the North American continent separated by the continental divide along the spine of the Rocky Mountains. The outcrop pattern of underlying geologic strata determines the geometry of a stream system. Tree-shaped, or dendritic, drainage patterns form when water flows randomly downhill without encountering geologic obstacles or conduits. Dendritic drainages are the most common and form when bedrock layers are horizontal. Trellis-shaped drainages develop in continental fold belts. Rectangular patterns are common in areas of fractured crystalline rocks, and streams flow down the sides of volcanoes in a radial pattern.

See also Alluvial systems; Drainage basins and drainage patterns; Canyon; Estuary; Hydrogeology; Sedimentation; Stream capacity and competence; Stream piracy; Stream valleys, channels, and floodplains