Discharge Controls on River Sinuosity: Analysis of a Small Modern Stream
Guangming Hu, Marjorie A. Chan, and Ziqiang Yuan
Normal river patterns suggest that slope gradient produces braided river channels upstream of a meandering river channel. A study of a small modern river system in the Uinta basin of Utah shows that discharge can influence channel sinuosity and morphology to produce initial meandering patterns that change later to braided patterns in the downflow direction. On a relatively flat (about 1-2°), modern, sandy pediment surface with scattered boulders, the small stream is informally divided into 3 portions: upstream, midstream, and downstream. The upstream segment is a high sinuous geometry dominated by both erosion (cutbanks) and deposition (point bars). The midstream is dominated by deposition (more small bars). The downstream section shows multistage erosional terraces, in the relatively straight channel system. Since the slopes of all three segments are consistently low, the key controlling factor of the channel patterns is the discharge. In the upstream, flood discharge occurs with large amplitude variation, and strong power that erodes the cutbank to produce sinuosity. Here, sand laterally accretes to form point bars. In the midstream segment, the current power decreases to form the braided river. A strong flood can cut off the point bar completely, which causes the channel to widen with a quickly corresponding current power decrease. Then, sediment from the upstream portion is unloaded in the midstream segment. Unloaded clastics can protect the bank in the midstream, and the current with low power reduces the erosion to the bank, which keeps the channel to a low sinuous geometry in the midstream. After the water drops its load, becomes 'clear' and reaches downstream, the lower current power is helpless to reform the river geometry. Thus, the downstream channel segment keeps a lower sinuous geometry, even straight partially. Simultaneously, little fine clastics (e.g. mud, silt) are deposited and multistage erosional terraces are formed during the flood regression. This stream example demonstrates the subtleties of stream flow and the importance of discharge. Similar effects are modeled in some experimental stream tables. Although it is difficult to scale this example up to large river systems that carve the geomorphic landscape, this shows how river geometries can vary from the traditional patterns attributed to gradient.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013