Restoring Alternate Bar Sequences in Large Rivers: Flow, Sediment, and Elbow Room
Abstract
Large river management must rely on crucial premises that the river channel is a product of its watershed's hydrology and the space within which it moves, and that native plant and animal species have evolved with, and still depend on, the natural annual hydrograph. While interesting academically, these premises may appear to offer very limited utility as management tools, given few large rivers in the Pacific Northwest and elsewhere have escaped major hydrologic and geomorphic change. The opposite, however, is likely true. Without reconstructing and incorporating an historical perspective, large river restoration will fail. Healthy river ecosystems can be achieved without completely restoring the natural unregulated hydrology, sediment supply regime, and original migration corridor. Usually there is little choice. An historical perspective identifies crucial threshold streamflow, sediment, and corridor processes driving a healthy river ecosystem. Alternating point bar sequences are the building blocks of alluvial rivers that also function as the physical template for creating abundant, high quality habitat for fish, amphibians, and riparian plant. While alternating bars can be created with bulldozers, their evolution, longevity, and quality will endure only if key geomorphic and biological threshold processes are provided by naturally variable annual hydrographs. An historical perspective therefore must identify how unregulated annual hydrographs once created and fostered dynamic alternating bar sequences. Restoration practitioners must then devise ways to restore these processes while purposefully cheating Mother Nature of some of her flow, sediment, and corridor width (elbow room). Two commonly prescribed high flow releases, flushing flows and the bankfull discharge, address several geomorphic requirements yet neither, or the two combined, can restore and maintain alternating bar sequences. Higher flood peak magnitudes from winter floods and spring snowmelt hydrographs are necessary, as are baseflows for limiting woody riparian encroachment. Vital to policymaking, alternate bars provide the often missing explicit and quantifiable link between prescribing high flows in regulated rivers and the benefits accrued from those releases.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.H44C..08T
- Keywords:
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- 1808 Dams;
- 1813 Eco-hydrology;
- 1821 Floods;
- 1834 Human impacts;
- 1856 River channels (0483;
- 0744)