Fluvial Geomorphology

Science Center Objects

An understanding of river- and stream-channel geomorphic responses to various human-caused and natural disturbances is important for effective management, conservation, and rehabilitation of rivers and streams to accommodate multiple, often conflicting, needs. Channel changes may have implications for the protection of property and structures, water supply, navigation, and habitat. The channel-bank erosion that accompanies natural channel migration on a flood plain represents a constant threat to property and structures located in or near the channel. Various human-caused and natural disturbances introduce additional instability to which rivers and streams adjust. Human-caused disturbances include reservoirs, channelization, in-channel sand and gravel extraction, and urbanization. A common natural disturbance is a flood. Possible geomorphic responses of a channel to disturbances include channel-bed degradation (erosion), channel-bed aggradation (deposition of material), channel widening, and channel straightening. These adjustments represent the channel’s attempt to establish a new approximate equilibrium condition.

bank erosion
Erosion along Stranger Creek has reached a house along the bank.(Public domain.)

Channel adjustments are a concern for several reasons. A substantial lowering of the channel bed poses an immediate threat to bridge pier foundations as well as buried pipelines and cables. In addition, substantial bed lowering increases bank height and bank instability that may trigger channel widening. Channel aggradation raises the bed elevation, reduces channel capacity, and increases the likelihood of flooding. Any channel-bed changes that occur on the main-stem rivers and streams also may migrate upstream on the tributaries where additional property, structures, and habitat may be at risk. Finally, any long-term channel adjustment processes also may instigate or worsen local scour problems.

Geomorphic investigations conducted by the U.S. Geological Survey since 1995 have mostly focused on the response of river and stream channels to various types of natural and human-caused disturbances including floods, reservoir construction and operation, and channelization. Such studies document channel changes, reconstruct historical conditions, determine the causes of channel changes, estimate the rate of geomorphic processes, and, in some cases, can enable predictions of future channel changes. Methods have included the use of streamgage data, multidate aerial photography, and onsite data collection to determine the location, timing, magnitude, direction, duration, and rate of channel change.

unfinished bridge
Kansas River at Lecompton, Kansas, following the 1951 flood (reproduced from McCrae, 1954).(Public domain.)

Floods can dramatically alter river channels. The record-setting 1951 floods in eastern Kansas caused substantial changes in channel-bed elevation at several sites and substantial channel widening at some sites (Bowen and Juracek, 2011). Historical floods along the Missouri River (1952, 1993, 2011) caused pronounced changes in channel-bed elevation (Juracek, 2014). Channel-bed elevation changes caused by large floods can persist and may have a cumulative effect.

Reservoir construction and operation can have a substantial effect on downstream river morphology. Channel-bed erosion downstream from 24 large reservoirs in Kansas ranged from negligible for a few sites to a maximum of about nine feet for the Republican River downstream from Milford Dam in northeast Kansas (Juracek, 2001). A review of reservoir effects is provided by Juracek (2015).

Channelization can substantially alter the morphology of a stream. Along Soldier Creek in northeast Kansas, channelization caused tens of feet of channel widening and several feet of channel-bed degradation that migrated several miles upstream (Juracek, 2002, 2004).

graph of change in annual discharge
Change in river/stream stage for mean annual discharge (1,000 cubic feet per second) of Republican River below Milford Dam (gaging station 06857100, map index number 27, downstream from Milford Lake), 1963–97.(Public domain.)