Geomorphological response to system-scale river rehabilitation I: Sediment supply from a reconnected tributary

This paper is the first of a pair that report the findings of a river rehabilitation project centred on the reconnection of a formerly diverted headwater tributary (Ben Gill) to its main‐stem river (the River Ehen). The present paper describes the geomorphic evolution of the tributary in the 2 years following its reconnection, with a particular focus on assessing the volumes of sediment now being supplied to the main‐stem Ehen. Structure‐from‐Motion photogrammetry was used to produce Digital Elevation Models (DEMs) of the newly connected tributary, with successive DEMs compared to assess topographic changes in the channel and quantify volumes of material exported. 3D errors in the DEMs were small relative to the scour and fill observed in the channel (error 0.016–0.056 m compared to up to 1.7 m vertical change between consecutive surveys). Erosion was the dominant process in the tributary channel, though this varied spatially and temporally. Over the 2‐year period, an estimated minimum of 384 m3 of coarse sediment was exported from Ben Gill and delivered to the confluence zone, where a new bar feature developed as a result. This estimate is twice as high as earlier ones. Analysis of the growth of this bar suggested that much of the material supplied by Ben Gill remains here temporarily, with onward conveyance constrained by the competence of the regulated main‐stem. The work shows that, thanks to the reconnection, this small (0.55 km2) ephemeral tributary (flowing for only around 20% of the time) has become a key source of sediment for the main‐stem Ehen. The second in the pair of papers focuses on the geomorphic responses of the main‐stem to this renewed supply of sediment.