I've been looking at stream flow records during the last week to understand effects of human management and storms on stream flows in the James River basin. The graphs below show recent flows in the Jackson River, the stream which joins with the Cowpasture River to form the James, in the mountains of Alleghany County.
Releases of water from Gathright Dam control flows in the Jackson River and in the James River for a considerable distance downstream under many conditions. However, we can see that the effects of this recent storm has overridden the influence of the Dam. The first graph below shows that stream flows were maintained at a stable, low level below Gathright Dam through controlled releases by the Corps of Engineers which operates that dam. In contrast, the flows rose very sharply in the Jackson River at the stream gage below Dunlap Creek at Covington.
Flows rose very quickly in the Jackson during a 9 hour period, with an increase of over 400%. Several factors likely contributed to this drastic increase in flow over such a short period. Consistent rains in recent weeks have contributed to saturated soils so that much of the rainfall runs off the land rather than soaking into the ground and the steep mountain slopes in this area can contribute to rapid rises. Finally, there are fairly large percentages of impermeable surfaces (streets, parking lots, buildings, etc.) in the Covington area. To assess the relative contributions of these factors, we can look at storm-related flows during other times of year when the soil is drier and more likely to hold a larger percentage of rainfall that lands on vegetated areas.
I and others plan to make a long-term study of the effects of Gathright dam releases on the James River system. Wherever human actions and structures affect a natural system, we must periodically consider whether these factors can or should be altered to provide for better water quality, for competing human uses, and for the overall natural balance of the stream ecosystem.
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