Role of flood discharge in shaping stream geometry: Analysis of a small modern stream in the Uinta Basin, USA

摘要

This small modern river system is located on a relatively flat (about 1°–2°), unconsolidated sandy pediment surface in the Uinta Basin of Utah, USA, and it is with a scale of about 30?m long and 0.4–0.8?m wide, similar as a natural flume experiment model. The small stream is informally divided into upstream, midstream and downstream. The analysis shows that flood discharge influences channel sinuosity and morphology to produce an initial meandering pattern which is later changed to a braided and then a straight pattern in the downflow direction.The upstream segment has a high sinuous geometry dominated by both erosion (cutbanks) and deposition (point bars). In the resistance of sporadic vegetation rooting in banks, the upstream flood deviates its original direction, which results in the powerful flood intensively eroding the cutbank and accreting clastics to build point bars, and thus producing a high sinuous channel.The midstream is dominated by deposits (many small bars) with a moderate to low sinuosity. Due to the bad drainage of the high sinuous channel in the upstream, the strong flood can cut off the point bar completely or even surmount the levee in the last meandering upstream, which widens the channel suddenly with a quick decreasing current power. Then, the clastics from the upstream are unloaded in the midstream and form many small bars. Unloaded sediments protect the bank, and the low-power current brings a moderate erosion to the bank, which forms a moderate to low sinuous channel in the midstream.The downstream shows multistage erosional terraces in its relatively straight channels. After the midstream water drops its load, it becomes “clear” and reaches downstream, the lower current power is helpless to reform channel geometry. Thus, the downstream channel segment keeps a lower sinuous geometry, even straight partially. Small amounts of fine clastics are deposited, and simultaneously multistage terraces are formed due to regressive flood erosion.This stream example demonstrates the subtleties of stream flow and the importance of flood discharge in shaping the channel geometry. Although it is difficult to scale up this example to a large river system that carves geomorphic landscape, this case shows how river geometries vary from the traditional patterns due to different gradient.