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WWRC 94-02
Riparian Zone Changes Caused by Streamflow Augmentation


A previously ephemeral watercourse in southeastern Wyoming has been used to convey water for the City of Cheyenne, Wyoming, since 1985. This water conveyance strategy was intended to partially mitigate the effects of the City's interbasin water diversions by creating a perennial stream along the pipeline diversion route. Since the effects of using this mitigation strategy are unknown, this study was initiated to evaluate the riparian vegetation response to flow augmentation, the rate of channel formation, and the depth-to-groundwater relationships for sedge (Carex spp.), tufted hairgrass (Deschampsia cespitosa [L.] Beauv.), and slimstem reedgrass (Calamagrostis neglecta [Ehrh.] Gaertn.). Above-ground biomass, density, basal cover, and below- ground biomass were measured for the riparian vegetation. Channel formation was evaluated with field surveys for channel length and cross section width, and measuring time-of-travel for a fluorescent dye. Depth-to-groundwater suitability curves for plants were based on the groundwater depth for 10, 50, and 90% (D10, D50, D90) of the June through September growing season.

Streamflow augmentation elevated the groundwater level to within 0.21 m of the surface for 90% of the growing season in the unchannelized meadows. The elevated water level initially increased sedges (240 g.m-2 in 1986 to 350 g.m-2 in 1988), while tufted hairgrass decreased between 1986 and 1989 (18 to 3 g.m-2). Below-ground biomass decreased (4,900 to 3,400 g.m-2) during 4 years of flow augmentation. The proportion of channel increased from 24% (2,017m) of the study area length before flow augmentation to 41 % (3,446 m) by the sixth year of flow augmentation. Most of this channel formed by downcutting rather than by the upstream migration of abrupt breaks in channel gradient (nick points).

The optimum depth-to-groundwater for sedge biomass was a nearly constant 0.15m (D10 to D90 ) of standing water. Tufted hairgrass response was optimized when the depth-to-groundwater was between 0.17 and 0.29 m for D10, deeper than 1.23 m for D50 and deeper than 1.79m for D90. The optimum depth-to-groundwater for slimstem reedgrass biomass was not well defined, but density appeared to decrease if the groundwater depths were shallower than 1.05m for D10, 1.34m for D50, and 1.81 m for D90.

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