River Response to Anthropogenic Modification: Channel Steepening and Gravel Front Fading in an Incising River

While most of the world's large rivers are heavily engineered, channel response to engineering measures on decadal to century and several 100 km scales is scarcely documented. We investigate the response of the Lower Rhine River (Germany-Netherlands) to engineering measures, in terms of channel slope and bed surface grain size. Field data show domain-wide incision, primarily associated with extensive channel narrowing. Remarkably, the channel slope has increased in the upstream end, which is uncommon under degradational conditions. We attribute the observed response to two competing mechanisms: bedrock at the upstream boundary increases the channel slope over the upstream part of the alluvial reach to compensate for the reduction of net annual sediment mobility, and extensive channel narrowing reduces the equilibrium slope. Another striking feature is the advance and flattening of the gravel-sand transition, suggesting its gradual fading due to an increasingly reduced slope difference between the gravel and sand reaches.

Automated summary

The study investigates the response of the Lower Rhine River to engineering measures, focusing on changes in channel slope and bed surface grain size. The observed response is attributed to two competing mechanisms: bedrock at the upstream boundary increases channel slope, while extensive channel narrowing reduces the equilibrium slope. Additionally, the gravel-sand transition zone is advancing and flattening, indicating a gradual fading due to decreased slope difference between the gravel and sand reaches.