A SMALL DAM WITH A BIG IMPACT: GEOMORPHOLOGY AND ECOLOGY OF TWO SECOND-ORDER STREAMS IN CENTRAL VIRGINIA

Dams have been shown to alter the geomorphology and ecology of streams. Effects of damming large rivers such as the Colorado and Columbia have been intensively studied, but the impact of small dams on small streams is not as well understood despite the fact that there are nearly 18,000 reservoirs with a volume of 50,000-100,000 m³ in the U.S. alone. This study compares two second-order streams in small (3-4 km²) adjacent watersheds on the campus of Sweet Briar College in the foothills of the Blue Ridge Mountains in central Virginia. One stream has been dammed for a little over 100 years, and the other is free-flowing. Although stream bank material, bedrock geology, and topography are virtually identical in the two watersheds, the particle-size distribution of bed material in riffles is very different. The median grain size in the dammed stream is twice as large as in the free-flowing stream, and sand is less abundant. Water temperature is generally higher in the dammed stream despite similar forest cover and shading. Flow regime in the two creeks is fairly similar. Striking differences are observed in fish and macroinvertebrate species and abundance, and indices of biodiversity show that stream ecology has been significantly impacted by the presence of the dam. The free-flowing creek has lower numbers of organisms but shows all the ecological characteristics of a healthy, cold-water mountain stream. The dammed creek, on the other hand, contains a higher percentage of pollution-tolerant species, and the fish population is dominated by introduced lake species. Although the complex interactions between water, sediment, and organisms in these creeks have not yet been fully explored, these findings show that small dams can have a major impact on lower order streams.