Setting environmental flows downstream of hydropower dams is widely recognized as important, particularly in Alpine regions. However, the required flows are strongly influenced by the effects of the physical environment of the downstream river. Here, we show how unmanned aerial vehicle (UAV)-based structure-from-motion multiview stereo (SfM-MVS) photogrammetry allows for incorporation of such effects through determination of spatially distributed patterns of key physical parameters (e.g., bed shear stress, bed grain size) and how they condition available stream habitat. This is illustrated for a dam-impacted Alpine stream, testing whether modification of the dam’s annual flushing flow could achieve the desired downstream environmental improvement. In detail, we found that (1) flood peaks in the pilot study were larger than needed, (2) only a single flood peak was necessary, (3) sediment coarsening was likely being impacted by supply from nonregulated tributaries, often overlooked, and (4) a lower-magnitude but longer-duration rinsing flow after flushing is valuable for the system. These findings were enabled by the spatially rich geospatial datasets produced by UAV-based SfM-MVS photogrammetry. Both modeling of river erosion and deposition and river habitat may be revolutionized by these developments in remote sensing. However, it is combination with more traditional and temporarily rich monitoring that allows their full potential to be realized.

<div> <p>Environmental policies have the purpose to protect ecosystems in their structure and function to maintain the ecosystem services they provide. They are based on scientific knowledge at the time they are established, and rarely are those assumptions revisited or is the effectiveness of these policies in protecting or promoting a particular ecosystem service tested. In this study, we revisit the first Swiss Federal Forest Law which protects mountain forests as a means of protection from natural hazards. It was established in 1876 following catastrophic flood events to preserve and restore the protective service of mountain forests by prohibiting clear-cutting and an excessive use of forests. Here, we provide a conceptual and methodological framework to explore the effects of the Forest Law on flood occurrence based on insights from preliminary results of a feasibility study. For the conceptual framework, we summarize the current scientific knowledge on i) forest effects on hydrological regimes and their protection service against floods, ii) reasons for reforestation in mountains and how the law may have contributed, and iii) other watershed changes affecting both reforestation and the forest-runoff interaction. We then develop the methodological framework based on insights from a case study on the Upper Rhone catchments, which serves as a prototype of an interdisciplinary methodological approach to answer the question of whether a forest protection law can serve as a means of flood protection. We explore the feasibility of answering this question given data are at different scales and resolutions. We suggest modeling to fill data gaps and discuss collaboration among natural and social sciences. Specifically, we propose that both natural and social scientists need to collaborate, with frequent exchange, to collect the data necessary to evaluate the relationship between legal forest protection and flood occurrence. We found an environmental historian is needed to evaluate if changes in forest cover can be attributed to mandates by the law, or rather cultural and societal developments. Further, a forest scientist or engineer in collaboration with a hydrologist will need to adapt and improve hydrological models that specifically include forest cover and structure. All scientists need to collaborate to find the information on historical and current forest cover (e.g., maps, postcards, orthophotos) and floods (e.g., archival documents, journal, newspapers, hydrological stations). Our case study indicates that data to answer the overarching question may be available and emphasizes the necessity of a true interdisciplinary approach allowing for consideration and combination of a variety of data sources and different temporal and spatial scales. The interdisciplinary framework we developed can serve as example for other ecosystem services, where similar questions on the effects of environmental practices and policies arise.</p> </div>

We are of the opinion that old environmental policies that are based on scientific knowledge at the time they are established need to be revisited in terms of the current knowledge and the effectiveness of these policies in protecting or promoting a particular ecosystem service. Here we use the first Swiss Federal Forest Law (1876) as a case example, which was established to protect mountain forests as a natural means of protection against natural hazards, particularly floods. We briefly summarize the current relevant scientific knowledge on i) reasons for reforestation in mountains and how the law may have contributed, ii) forest effects on hydrological regimes and their protection service against floods, and iii) other watershed changes affecting both reforestation and the forest-runoff interaction. We then present insights from a case study on the Upper Rhone catchments, which lead us to develop a methodological approach based on interdisciplinary collaboration among natural and social sciences to gain the needed data to answer the question of whether a forest protection law can serve as a means of flood protection. Specifically, we found that a means of data interpolation is key to answering this question given data are at different scales and resolutions, and suggest modeling methods to fill gaps. Such methods and collaborations are key for basing environmental laws and policies in current scientific knowledge and effectively manage ecosystems and their services.

We are of the opinion that environmental policies that are based on scientific knowledge at the time they are established need to be revisited in terms of the current knowledge and the effectiveness of these policies in protecting or promoting a particular ecosystem service. Here we use the first Swiss Federal Forest Law (1876) as a case example, which was established to protect mountain forests as a natural means of protection against natural hazards, particularly floods. We briefly summarize the current relevant scientific knowledge on (i) reasons for reforestation in mountains and how the law may have contributed, (ii) forest effects on hydrological regimes and their protection service against floods, and (iii) other watershed changes affecting both reforestation and the forest-runoff interaction. We then present insights from a case study on the Upper Rhone catchment, which lead us to develop a methodological approach based on interdisciplinary collaboration among social and natural sciences to gain the needed data to answer the question of whether a forest protection law can serve as a means of flood protection. Specifically, we found that a data interpolation method is key to answering this question given data are at different scales and resolutions and suggest modeling to fill gaps. Such methods and collaborations are key for basing environmental laws and policies in current scientific knowledge and effectively manage ecosystems and their services.