The legacy of river channel modification in wadeable,lowland rivers: Exploring overdeep rivers in England

Many lowland fluvial systems are suspected to possess a morphological legacy from a long history of channel modifications as a consequence of limited energy and sediment supply to facilitate recovery. We explore the extent of such modifications using a regionally extensive dataset of physical habitat surveys compiled by non-specialist surveyors. Representative photographs for each surveyed site were used to quality check channel width, depth and bed grain size information derived from Modular River Physical (MoRPh) surveys. Following checking, 1659 surveys were retained for analysis from alluvial sites, almost entirely in England. The photographs were also inspected for evidence of clear ‘overdeepening’ that would preclude frequent overtopping of the lower bank top. Results indicated that almost one-third of the sites were overdeepened, that width-to-depth ratios defined using the active bed width showed stronger discrimination of overdeepening than bankfull width, that highly statistically significant identification of overdeepened channels was possible in channels up to 10 m wide and with only minimal differences attributable to channel bed materials. Stepwise regression analysis estimated relationships between channel width-to-depth ratios and channel size for overdeepened and non-overdeepened channels. We demonstrate that large data sets collected by numerous non-specialist surveyors can, with careful filtering, generate statistically robust results of geomorphological value over areas larger than is otherwise practicable. Furthermore, we reveal a notable legacy of overdeepening in the analysed lowland rivers, which presents a significant ‘hydromorphological’ management challenge.