The heterogeneity of the hydromorphological responses of a stream to the urbanization of its basin

Morphological adjustment mechanisms and controls of small urban rivers are rarely studied especially in France. We propose an original hydrogeomorphological approach based on a detailed study of a whole catchment using field data acquired systematically all along the river: (i) channel cross-sections; (ii) riffle-pool sequences and (iii) obstacles to flow and rainwater drainage pipes. The entire length of the river was surveyed and the channel geometry (depth and width at bankfull) was measured on a cross-section every 180 m or at each riffle (depending on which of the two intervals happened to be the shortest). The 357 transects thus established constitute as many elementary units of measurement (EUM) of the hydrographic network. Nine hydromorphological variables were used as a basis to classify the profiles (principal component analysis/hierarchical clustering analysis). The types were compared with 12 anthropogenic constraining variables to explain longitudinal morphological variability and to link morphology to pressures on this scale. The results showed an average incision of 0.50 m (locally over 1 m), an average widening of nearly 1 m (locally over 3 m), and heterogeneous distribution of the riffle-pool sequences (although not complete disappearance). Systematic analysis of within-channel forms revealed high morphological diversity, the absence of a linear relationship between the rate of urbanization and incision/erosion, and the absence of longitudinal changes in response to urbanization. The constraints identified are commonly observed in urbanized basins, but the variety of responses to the constraints makes it difficult to understand the system as a whole and prevents uniform ecological restoration along the entire river.