Over 200,000 kilometers of free-flowing river habitat in Europe is altered due to impoundments

Abstract

European rivers are disconnected by more than one million man-made barriers that physically limit aquatic species migration and contribute to modification of freshwater habitats. Here, a Conceptual Habitat Alteration Model for Ponding is developed to aid in evaluating the effects of impoundments on fish habitats. Fish communities present in rivers with low human impact and their broad environmental settings enable classification of European rivers into 15 macrohabitat types. These classifications, together with the estimated fish sensitivity to alteration of their habitat are used for assessing the impacts of six main barrier types (dams, weirs, sluices, culverts, fords, and ramps). Our results indicate that over 200,000 km or 10% of previously free-flowing river habitat has been altered due to impoundments. Although they appear less frequently, dams, weirs and sluices cause much more habitat alteration than the other types. Their impact is regionally diverse, which is a function of barrier height, type and density, as well as biogeographical location. This work allows us to foresee what potential environmental gain or loss can be expected with planned barrier management actions in rivers, and to prioritize management actions.

Fig. 1. Conceptual Habitat Alteration Model for Ponding (CHAMP).

Steps involved in the model building process.

Fig. 2. Structure of expected fish communities presented as a proportion of 11 Habitat Use Guilds for each FCMacHT.

Guilds are ordered from more rheophilic/less tolerant to generalist/more tolerant species. For FCMacHT descriptors, refer to Fig. 3 and Supplementary Information, Box 2.

Fig. 3. Fish Community Macrohabitat Types (FCMacHT) in European running waters.

Data source: CCM v2.1 for river segments, ECRINS for country borders.

Fig. 4. Fish community sensitivity to impounding.

Highest sensitivity: fish communities where >55% of preferred habitat characteristics may potentially be affected by in-river barriers and Lowest if less than 45% of those are. Moderate sensitivity is for the values in between. It is calculated as a sum of Habitat Preferences weighted by guild proportions in the fish community over the maximum. The threshold classification method is Jenks Natural Breaks.

Fig. 5. Estimated barrier habitat impact across European rivers with respect to barrier type.

Results can help inform where the building of new structures is likely to cause large habitat alteration or where restoration measures are likely to produce the largest habitat gains. Each panel refers to an individual barrier type. Classification based on wRHp values. Purple—severe habitat alteration (≤25%), red—major habitat alteration (26–50%), orange—substantial habitat alteration (51–75%), yellow—moderate habitat alteration (76–90%), blue—low habitat alteration (>90%).

Fig. 6. Projected barrier-level impounding impacts in European catchments.

Median value of weighted riverine habitat proportion (wRHp) of barriers in the same catchment (refer to Table 1) was used. The inset figure’s extent is marked with a rectangle in the main map. Points in the inset figure represent individual recorded barriers. Regions with low per-barrier impact in Central Europe indicate a relatively high number of small barriers (culverts, fords, and ramps) with low individual impact contained in national registers, while major per-barrier impact in other parts of Europe indicates the prevalence of a relatively high number of large barriers and sensitive fish communities. Data sources: AMBER Barrier Atlas (acc. Nov. 2020) for barrier locations, CCM v2.1 for river segments and catchments.

Fig. 7. Hypothetical example of application of our results.

We adapted restoration alternatives analysis to demonstrate the impact of five barriers in a watershed located on the main stem of the river. It demonstrates how much each barrier alters the free-flowing riverine habitat (1-wRHp) and how much of the upstream river length is no longer available for migrating fish. Barrier 5 is the closest to the river mouth.