Tuesday, September 9, 2008
Rhetorical Precis Trush
William J Trush, Scott M. McBain, and Luna B. Leopold’s article, “Attributes of an Alluvial River and Their Relation to Water Policy and Management” (8/15/2000), explains the 10 attributes of alluvial rivers and how these attributes should guide the examination of alluvial rivers so that the restoration of alluvial processes may successfully occur downstream from a dam, or preserve alluvial streams below proposed dam sites. Trush, McBain and Leopold identify the growing body of evidence, experiment and theory concerning the geomorphic processes that form and maintain alluvial river ecosystems which need to be taken into consideration when stream restoration of an existing area or stream management downriver of a dam is implemented. In order to help river managers identify desired processes by use of “a minimal checklist” of critical geomorphic and ecological processes derived from field observation and experimentation, allowing for use of charting and evaluation strategies for restoring and preserving alluvial ecosystems, and then help prescribe the necessary impetuses based on useful empirical relationships, the authors’ purpose is to point out ways in which to restore alluvial streams. Trush, McBain and Leopold created this document for use by scientists, biologists, ecologists, as well as policy makers for appreciating, interpreting and restoring the complexity of alluvial river ecosystems.
Subscribe to:
Post Comments (Atom)
3 comments:
I thought this was a really good paper! It condensed the ground- breaking work Luna Leopold and his colleagues accomplished in the last 40+ years into a few pages. However, what do we lose in this simplification? I would like to know more about sediment dynamics when discharge approaches and exceeds bankfull. Does substrate move one peice at a time in a predictable sequence dependent on density? Or does discharge hit a point where the entire bed mobilizes? How should we think about a river that is not alluvial?
Trush et al (2000) outline ten attributes that describe alluvial rivers. These attributes are designed to provide criteria for managers who wish to restore the natural geomorphic processes that affect rivers. These attributes are placed in the context of remediating rivers that have been altered by dams, without having to remove the dam. The attributes require managers to view alluvial rivers as dynamic systems that are governed by interrelated geomorphic processes. The authors attest that the natural hydrologic regimes function to create alluvial channels that meander, scour, migrate, and transport sediment. They also state that natural alluvial rivers regularly enter the floodplain and create distinct riparian vegetation communities. The authors write that these natural geomorphic processes create complex habitat that supports robust ecosystems. Trush et al (2000) cite the example of the Trinity River, CA, which was impounded to provide power and water supply for agriculture and municipal purposes. They show that alterations in the natural alluvial hydrology resulted in altered channel characteristics and reduced salmon stocks (due to loss of spawning habitat). They propose a plan of calculated discharge that emulates spring snow melt and attempts to balance the sediment budget. They believe that this will help to restore the alternating bar channel sequence, and thus restore salmon spawning habitat that was lost when the dams were installed. The authors conclude that if alluvial rivers are to be restored without sacrificing the anthropogenic benefits of dams, managers will have to settle for a smaller versions of alluvial rivers that receive less sediment input and operate with less-than-natural hydrology.
In “Attributes of an alluvial river and their relation to water policy and management,” Trush, McBain, and Leopold outline ten ways to describe, assess, maintain, and restore alluvial river systems:
1. Alternate bar sequence is the main patterning of alluvial systems in a geomorphic and ecological context.
2. Hydrographs should show proper elements for maintaining an alluvial system.
3. Mobilization occurs on the channelbed surface via a flood event every 1-2 years.
4. Alternate bars need to be periodically rejuvenated through mobilization; this process happens by scour and redeposition after scour.
5. The ratio of sediments, coarse and fine has an equal input v. export.
6. Lateral migration of channels over periods of time, specifically during flood events, causes channel width maintenance.
7. Inundation of flood plains via a flood event maintains critical processes.
8. Streams have larger flood events (10-20 year floods) that maintain and restore different stream and floodplain conditions.
9. Natural events in a stream support riparian zones.
10. Groundwater is recharged during flood events.
The Trinity River at Lewiston shows an ideal example of what happens to a stream when a dam is built and flow is diverted. Within a short period of time, the dynamic equilibrium of the river was thrown off; Trinity River still retains major problems today, even with the increased dam efforts to restore natural conditions.
Citation:
Trush, W.J., S.M. McBain, and L.B. Leopold. 2000. Attributes of an alluvial river and their relation to water policy and management. Proceedings of the National Academy of Sciences of the United States of America 97(22): 11858-11863.
Post a Comment