A systematic analysis of in-stream mixing lengths in two mid-mountain headwater streams: incongruities, insights and implications

Estimating in-stream mixing lengths is important in the context of salt dilution gauging, but also in the context of stream water quality assessments. Underestimating the mixing length can lead to large errors and misinterpretations of your data.  Playing it safe and going with an overly long mixing length can also introduce errors. For example, the underlying assumption of salt dilution gauging of conservation of mass might be violated when stream losses become significant. However, despite their importance, mixing length estimates are often only based on experience or empirical equations.

In this study we estimated the mixing lengths for 10 different stream reaches in two mid-mountain headwater streams. Three tracer experiments were carried out at each stream reach: dye injection and salt injections, here both as slug and constant rate injections. Breakthrough curves of the salt injections were monitored using 20 electric conductivity sensors. The results of the tracer injections are then compared to other common methods of mixing length estimation and the implications are discussed.