A STUDY OF WATER MASS MIXING AT THE CONFLUENCE OF LOW ORDER STREAMS IN CENTRAL VERMONT

When water masses meet, as in the case of a stream confluence, they can remain unmixed or only partially mixed for long distances, which can be described by mixing models. The degree of mixing over a short distance was studied for two mountain streams in central Vermont; the Dog River and Cox Brook, the former being the larger. The Dog River has a gradient of 0.0067, the Cox Brook, 0.017. Both flow over cobble gravel and bedrock and there is a submerged sand bar at the confluence. At base flow channel depths are 1-3 ft. The two streams drain distinctively different geologic terrains, the Dog River watershed in part comprising calcareous metasediments and the Cox Brook watershed comprising siliceous moderate-high grade metamorphics. Therefore, geochemical signatures can be used to evaluate the degree of mixing.

Detailed sampling transects were established upstream and downstream of the confluence, and sampling was conducted at base flow and at moderate discharge (~30cfs, ~100 cfs). At base flow, the water bodies differ in pH, temperature, dissolved oxygen content, and dissolved load (TDS defined as Ca, Mg, Na, K, Si, Al). At moderate discharge, dilution of the dissolved load is apparent but the water bodies remain distinguishable above the confluence. Below the confluence, at base flow, the water bodies are mixed within the first 16 ft, but there is some indication that horizontal stratification has occurred, and by 40 ft downstream the water mass is mixed but in a gradational manner across the channel. At moderate discharge the two water masses remain only partially mixed for at least 40 ft. Modeling, based on gradient, channel parameters, and velocity suggests a complete lateral mixing distance of over 100 ft., and this may be the case, but for moderate discharge only. At base flow conditions, nearly complete mixing apparently occurs over a very short distance (less than 20 ft.).