The third dimension in river restoration: how anthropogenic disturbance changes boundary conditions for ecological mitigation

Stream restoration can reduce nitrate levels in agricultural landscapes

The EU Water Framework Directive (WFD) has emphasized that altered stream/river morphology and diffuse pollution are the two major pressures faced by European water bodies at catchment scales. Increasing efforts have been directed toward restoration to meet WFD standards for ecological health, but this work has achieved limited success. One challenge is that little is known about how morphological changes (i.e., re-meandering) may affect nitrate retention within whole stream networks. We investigated this issue in the well-monitored Bode catchment (3200 km2) in central Germany. First, we implemented a fully distributed process-based mHM-Nitrate model, exploring its performance over the period from 2015 to 2018. Second, we simulated the effects of restoring more natural stream morphology (i.e., increasing sinuosity) on nitrate retention. The mHM-Nitrate model performed well in replicating daily discharge and nitrate concentrations (median Kling-Gupta values of 0.78 and 0.74, respectively). Within the stream network, mean and standard deviation (SD) of gross nitrate retention efficiency was 5.1 ± 0.61 % and 74.7 ± 23.2 % in the winter and summer, respectively; this measure took into account both denitrification and assimilatory uptake. In the summer, the denitrification rate was about twice as high in a lowland sub-catchment dominated by agricultural lands as in a mountainous sub-catchment dominated by forested areas (median ± SD of 204 ± 22.6 and 102 ± 22.1 mg N m-2 d-1, respectively). Similarly, in the same season, the assimilatory uptake rate was approximately five times higher in streams surrounded by lowland agricultural areas than in streams in higher-elevation, forested areas (median ± SD of 200 ± 27.1 and 39.1 ± 8.7 mg N m-2 d-1, respectively). This suggests that restoration strategies targeted at lowland agricultural areas may have a greater potential for increasing nitrate retention. In our simulation, restoring stream sinuosity was found to increase net nitrate retention efficiency by up to 25.4 ± 5.3 %; greater effects were seen in small streams. Taken together, our results indicate that restoration efforts should consider augmenting stream sinuosity to increase nitrate retention and decrease nitrate concentrations at the catchment scale.

Morphometric delineation of administrative boundaries and classification of threatened categories of small watersheds in transboundary rivers

The ecological conservation of large rivers is impossible unless immediate attention is given to protecting their small tributaries at local levels. The natural boundaries of large river basins are shrinking because their tributaries and streams of different orders are disappearing at an unprecedented rate. Delineation of the fixed administrative boundaries (AB) to protect the natural boundary of small rivers and their classification into appropriate threatened categories, the present study was carried out on the 54.08 km long Banki River in the Ganga River basin. The > 70% irreversible loss in the number of streams (Nu), length of streams (Lu), and drainage density (Dd) resulted in the conversion of the 6th order Banki into the 4th order river. The extreme morphometric changes result in the Banki watershed being under the "Critically Endangered" category. The drainage density ratio (DdR) and mean stream width (M_sw) were used to determine the width of AB (W_AB). The "River Red List Categories and Criteria" are being proposed to strengthen global initiatives at the local levels to protect and conserve inland water bodies and transboundary rivers.