Geomorphological response to system-scale river rehabilitation II: Main-stem channel adjustments following reconnection of an ephemeral tributary

Abstract

This paper describes changes in bed morpho-dynamics and topography in the River Ehen, a regulated river in NW England (i.e., temperate climate) following a rehabilitation project that reconnected a formerly diverted headwater sub-catchment back to its main-stem. Sediment grain-size distributions in the Ehen changed subtly and in rather complex ways following the reconnection. Changes were most evident in the riffle morphological unit, where gravel-sized material accumulated in the first 2 years after the reconnection. All morphological units initially experienced an addition of fine sediment (size <8 mm) but by the end of the study the proportion of fine material in the bed matrix had returned to pre-reconnection levels. Topographic changes were evident in some units, with net aggradation in the riffle and scour in the plane bed; there was no detectable change in the pool. Albeit limited, there was evidence of an increase in bed mobility, with field observations indicating that the new sediment is moving over the top of the largely static existing pavement, rather than interacting with it. Despite numerous uncertainties related mainly to the ephemeral nature of the tributary and, consequently, how much sediment it would deliver, evidence suggests that the main project objective is being met: there is a renewed supply of sediment now being delivered to the main-stem Ehen at times and in quantities that are controlled by natural processes. Nevertheless, the river is still best considered to be in an adjustment phase, so assessment of its long term response to the reconnection requires continued monitoring.