Hydrologic scenarios for floodplain building in a vertically accreting, suspended sediment river
Abstract
Recent advances in dendrogeomorphology allow for precise age constraint on rates of floodplain building. We applied these techniques in our effort to evaluate the relative roles of flow regulation and invasive riparian vegetation in accelerating the rate of vertical accretion and channel narrowing of the dam-regulated upper Green River in Dinosaur National Monument, Colorado and Utah. Four large trenches were excavated into alluvial deposits that are now inundated by either common post-dam floods or rare post-dam high releases. We dated individual stratigraphic units using the recently developed stem-burial method (Friedman et al. 2005), as well as traditional cross-dating methods. These excavations indicate that episodic large floods with pre- dam recurrences of 5 to 10 years may vertically aggrade floodplains by up to 1.0 m. Smaller, more frequent, floods progressively create inset alluvial deposits that are subsequently colonized by native or non-native woody shrubs and trees. These lower elevation deposits serve as growing surfaces for both native and non- native riparian plants as well as stable platforms for deposition during occasional large post-dam floods that exceed the capacity of the dam's power plant. This style of floodplain building results in channel narrowing and loss of aquatic habitats. In the case of the upper Green River, the majority of channel narrowing occurred during two isolated events: (1) a rare basin-wide flood in late winter 1962, shortly before completion of Flaming Gorge Dam and (2) the largest post-dam bypass flood in 1983. Thick deposits of these ages occur in each trench, and this pattern correlates with the sequence of channel narrowing described by Allred and Schmidt (1999) 320 km downstream at the USGS gage near Green River, Utah. Our results suggest infrequent, controlled, high-magnitude dam releases may be an ineffective means of channel rejuvenation and vegetation control along vertically accreting, suspended sediment rivers. Where post-dam rivers are in a condition of sediment surplus, large floods may further isolate unwanted riparian vegetation from lower, more frequently disturbed hydrologic environments by increasing the elevation of the higher alluvial deposits.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.H14D..04A
- Keywords:
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- 1813 Eco-hydrology;
- 1820 Floodplain dynamics;
- 1825 Geomorphology: fluvial (1625);
- 1851 Plant ecology (0476);
- 1861 Sedimentation (4863)