On secondary circulation, helical motion and Rozovskii-based analysis of time-averaged two-dimensional velocity fields at confluences

In this response to Lane et al. (1999) 'Time averaged flow structure in the central region of a stream confluence: a discussion', Earth Surface Processes and Landforms, 24, 361-367 we show that our method for decomposing the cross-stream velocity field into components associated with primary and secondary velocities, as defined by Rozovskii, is not flawed or misleading. Instead, it yields valuable information that illustrates the contribution of secondary circulation and, by inference, helical motion to the pattern of cross-stream flow at confluences. Lane et al.'s concern about our methodology can be attributed to their failure to distinguish clearly and consistently between secondary circulation and cross-stream discharge, a distinction that is central to our method of analysis, and to their inappropriate comparisons of velocity fields for the different frames of reference associated with secondary circulation and cross-stream discharge.