Effects of dam removal on river form and process

Presented at 2001 Ecological Society of America Annual Meeting 10aug01

[ Ecosystem responses to removal of low-head dams from two Wisconsin Rivers - below ]

Symposium #23: Dam removal as river restorations: Linking ecological, engineering, social, and legal perspectives.
Sponsored by ESA Aquatic Ecosystems Section
Organized by: D. D. Hart and N. L. Poff.
Friday, August 10, 2001. 8:00 AM to 12:00 PM. Madison Ballroom A

James Pizzuto
Dept. of Geology, University of Delaware

ABSTRACT- When a dam is removed, theoretical principles and empirical data suggest a predictable series of events: the stream will incise into the sediment fill behind the dam, sending a pulse of sediment downstream. This process will continue until a stable channel and floodplain have developed above the former dam site, and until the extra sediment stored in the downstream reach has been removed. The processes that drive these changes may be cautiously inferred from classic geomorphic field studies of incision and floodplain construction, and from recent field and laboratory studies of sediment pulses. Promising numerical models have recently been developed for some of these processes, but they are relatively untested. Even qualitative predictions of the geomorphic evolution of a stream valley at a particular removal site are not reliable at present. A variety of studies are needed, including 1) monitoring studies of dam removal processes under a variety of conditions, 2) controlled physical modeling with parallel development of predictive numerical models, 3) field studies specifically designed to test the predictive ability of qualitative and quantitative models, and 4) continuing efforts to quantify the transient responses of streams to changing discharge and sediment supply, particularly at the varied scales of interest to ecologists. Even if predictive geomorphic models can be developed, their application will be strongly influenced by the variety of engineering methods used in dam removal projects.

KEYWORDS- geomorphology, rivers, sediment transport, dam removal

Ecosystem responses to removal of low-head dams from two Wisconsin Rivers

Symposium #23: Dam removal as river restorations: Linking ecological, engineering, social, and legal perspectives.
Sponsored by ESA Aquatic Ecosystems Section
Organized by: D. D. Hart and N. L. Poff.
Friday, August 10, 2001. 8:00 AM to 12:00 PM. Madison Ballroom A

Emily Stanley¹, Martin Doyle².

University of Wisconsin, Madison, WI ¹, Purdue University, W. Lafayette, IN ²

ABSTRACT- Removal of low-head dams produces conspicuous changes in physical attributes of riverine ecosystems. Coupled chemical and biological responses may be equally dramatic, but in the absence of detailed research, are less obvious. Many dams targeted for removal in Wisconsin are old (100 years), and impoundments typically contain extensive sediment deposits. The fate of stored sediment represents a major research and management challenge in terms of upstream and downstream habitat alteration, as well as nutrient distribution and transformation. In this context, we have been investigating short-term changes in riverine nutrient and sediment transport following dam removal. Dam removal can shift impounded reaches from sediment and nutrient (N and P) sinks to sources. Changes in P retention are associated with mobilization and downstream transport of reservoir sediments, whereas N dynamics appear to be governed by loss of conditions suitable for denitrification. These findings raise several questions regarding nutrient transport in the context of dam removal, including: How long do formerly impounded reaches act as nutrient and sediment sources? How much of the nutrient pool stored in reservoir is mobilized to downstream reaches? What is the extent of downstream effects of sediment and nutrient inputs? How does the redistribution of sediment affect nutrient retention in downstream reaches? Given current concerns about riverine nutrient transport and retention, there is a critical need to address these questions, particularly in nutrient-enriched riverine systems.

KEYWORDS- dams, nitrogen, phosphorus, restoration