Sediments pathways to small rivers in loamy agricultural region and where to find them

Fine sediments cause a wide range of damages to rivers, impacting morphology and aquatic communities. Sediments in rivers come from bank erosion and catchment erosion. Tackling erosion in small agricultural river catchments is essential because this part of the landscape is the production zone: where erosion and sediment production take place. Analysis of catchments area is crucial because river ecosystems are closely linked to their watershed and their land use.

Measures to reduce erosion can be taken within the watershed but this research focuses on the riparian zone associated with small downstream (semi)-permanent rivers (catchments > 1 km²). Indeed, these small rivers are where the characteristics of the riparian zone exert a strong control on the aquatic environment, notably by filtering sediments from the land. As erosion is not uniform across the landscape, the aim of the research is to identify where riparian vegetation should be used to mitigate sediment transfer and deposition. The objectives of this research are twofold: i) to locate small agricultural catchments prone to sediment transfer to the river, ii) to understand how riparian zones can better control sediment transfer from the land to the river.

We conducted our analysis in the erosion-prone loess region of Wallonia (southern Belgium), where about 65% of the surface is used for agriculture. To identify sites of sediment transfer in this region, catchments areas of at least 0.2 km² with an outlet in small rivers (> 1 km²) are drawn. To describe the erosion process, soil type, slope, land use and agricultural background are analysed for each identified catchment. The land use data allow to exclude watersheds that are too urban or too impermeable by roads or railways. Among the selected catchments, the intensity with which the crop can favour sediment production is analysed based on crop history (from 2015 to 2022), with a focus on erosion-prone crops like maize, sugar beet or potatoes. The riparian zone associated with the outlet of these small catchments is described using several parameters: the height above nearest drainage, the size of the downward river, and the angle at which the concentrated flow enters the river. The width, the height, the composition and the continuity of the riparian zone around the confluence are also analysed. Sediment deposition signal at the outlet is investigated using the difference between two regional LiDAR DEMs acquired in 2011 and 2022. We expect the catchment characteristics to determine the intensity of the deposition process. We also compare the physical parameters of the riparian zone with the deposition intensity to assess its sediment filtering ecosystem service. The presentation will show the current progress of this research.