Seasonal Use of Missouri River Reservoir Deltas by Fishes

Reservoir aging presents substantial challenges to natural resource managers. While most reservoirs were designed to fill with sediment slowly, many have accumulated enough sediment to require management (i.e., removal of sediment by flushing, piping, or dredging). However, novel aquatic habitats have developed as a result of sedimentation (e.g., reservoir deltas), and recent research suggests that these habitats are important to native fishes and support greater fish diversity than other reservoir habitats. As water resources engineers evaluate sediment management techniques, it is important that fisheries biologists understand the use of these habitats by fishes and the potential impacts of their removal. Given the limited amount of information regarding the development of these habitats and their use by fishes, I studied recent changes in the littoral habitats of reservoirs on the Missouri River and collected fishes in deltas habitats. I focused on two research topics during this study: 1) recent changes in littoral habitats resulting from sedimentation and shoreline erosion, and 2) the seasonal use of delta habitats by fishes. Results from this study will contribute to the understanding of aging reservoir ecosystems and help guide sediment management. Although water resources engineers have monitored the aging of Missouri River reservoirs since their construction, fisheries biologists have only periodically evaluated the resulting changes in aquatic habitat. In Lake Francis Case, South Dakota, changes have not been evaluated since the 1970’s. Thus, I assessed changes in the littoral habitats of Lake Francis Case from the 1990’s (1991-1998) to 2008 using aerial imagery and geographic information systems (GIS) technology. Reservoir deltas (n=2) and embayments (n=100) were digitized to create polygons suitable for calculating geometries and I quantified changes in littoral habitats using embayment surface area, shoreline development and area of offshore features. I also quantified the growth of deltas to help describe recent patterns in development of these features. In general, embayments in the upper reservoir decreased in surface area, increased in shoreline development and gained offshore features, while embayments in the lower reservoir increased in surface area, decreased in shoreline development and lost offshore features. Changes in embayment surface area ranged from a 2.25% average annual decrease, to a 3.19% average annual increase, and differed by river kilometer (r = 0.24; P = 0.02). Sedimentation and the expansion of vegetation in embayments resulted in losses of surface area, while shoreline erosion resulted in gains of surface area. Changes in shoreline development followed a similar pattern with upper embayments typically increasing in shoreline complexity, and lower embayments typically decreasing in shoreline complexity (r = 0.38; P \u3c 0.01). Increases in shoreline development were attributed to macrophytes colonizing the shoreline and decreases were attributed to shoreline erosion. Offshore features in all (n=13) but three embayments increased in surface area. Rates of surface area change ranged from an average annual increase of 0.36 ha to an average annual decrease of 0.01 ha. Increases in surface area of offshore features were attributed to the expansion of macrophytes, and the loss of surface area of three lower embayments was attributed to erosion. Between 1991 and 2008 the White River delta expanded by 144%, resulting in an average annual expansion rate of 8.60%, while the small delta at the confluence of Campbell Creek (upper) expanded by only 0.10%, resulting in an average annual expansion rate of \u3c0.01%. The differing expansion rates were likely a result tributary size and sediment load. Campbell Creek (upper) is a small intermittent stream whereas the White River is a large perennial river delivering considerable sediment to Lake Francis Case. Also, the rapid expansion of the White River delta was likely due to the colonization of the lower (southwestern) portion by macrophytes (e.g., cattails Typha spp.). This area will remain vegetated unless these macrophytes are submerged for an extended period of time and will help secure sediment on the delta. The White River delta will likely continue to expand downriver. Overall, the driving forces of habitat change in Lake Francis Case appear to be sedimentation (and the resulting expansion of macrophytes) and shoreline erosion. While the littoral habitats of lower embayments appear to be decreasing in complexity, littoral habitats in upper embayments and especially at the White River delta appear to be increasing in complexity. Sedimentation in these areas has enabled macrophytes to become established and is providing heterogeneous habitat unique in the reservoir. To assess the seasonal use of reservoir deltas by fishes, I collected fishes in delta habitats in 2008 and 2009 during four periods; April, June, August, October (spring, summer, late summer, and fall). Fishes were collected from the Niobrara River delta in Lewis and Clark Lake, the White River delta, and at a reservoir site in Lake Francis Case. I compared fish assemblages between two segments of the Niobrara River delta to better understand the spatial and temporal distribution of fishes within a large delta. I compared fish assemblages between the White River delta and the reservoir site to evaluate the potential importance of this delta to fishes in Lake Francis Case. Species richness and diversity did not differ between the segments of the Niobrara River delta in Lewis and Clark Lake, or the White River delta and the reservoir site in Lake Francis Case. The similarity of sites in Lake Francis Case challenges the trend previously described in Lewis and Clark Lake where delta sites supported greater richness and diversity than reservoir sites. If the White River delta develops a greater variety of habitats over time (e.g., interspersed fluvial habitat and slackwater habitat) it may ultimately support more species than reservoir habitats. Overall, catch per unit effort (CPUE) of delta fishes was variable, but several patterns were apparent. Age-0 fishes were collected in delta habitats in the late summer and fall, while juvenile fishes were collected throughout sampling, and adult fishes were most often collected in the spring and summer. Thus, reservoir deltas appear to provide nursery habitats for age-0 fishes, refuge for small-bodied fishes and pre-spawn habitat for adults. The development of deltas may help enhance fish production, as well as retain fishes reproduced in the reservoirs and these novel features may need to be conserved