Multi-year artificial flooding to restore a hydropower-regulated floodplain system: Balancing biodiversity needs and hydropower requirements

Artificial floods can be used to restore hydromorphological and ecological processes in floodplains downstream of dams, where altered flow and sediment regimes often reduce biodiversity and habitat dynamics. In the Sarine floodplain (Canton Fribourg) below the Rossens dam, a floodplain of national importance, decades of residual-flow management and bedload retention have led to stabilized channel conditions, loss of open gravel areas, excessive algal growth, riverbed clogging, and a macrozoobenthos community dominated by lentic taxa adapted to low-disturbance environments. To counteract these deficits, eleven artificial floods between 2016 and 2025, ranging from 75 to 650 m³/s, were released and monitored using in situ measurements and UAV surveys for long-term hydromorphological and ecological change detection.

The results show that artificial floods trigger immediate but short-term ecosystem responses. Lotic macrozoobenthos taxa adapted to variable flow conditions, particularly EPT groups, increased directly after larger flood events. Periphyton biomass declined even after small floods but regenerated rapidly in the absence of repeated disturbance. Morphological analyses revealed substantial erosion, redistribution of sediments, and the re-opening of gravel habitats during high-magnitude floods. These morphological changes are crucial for restoring habitat heterogeneity and promoting biodiversity.

However, isolated and infrequent floods are insufficient to generate sustainable hydromorphological and ecological improvements. Without regular disturbance, the system quickly returns to residual-flow conditions, limiting long-term gains in habitat quality and macrozoobenthos diversity. Detailed long-term analyses further indicate that flood frequency and duration, rather than magnitude alone, are key drivers controlling changes in macrozoobenthos functional composition and the balance between lentic and lotic assemblages.

Therefore, an adaptive management program was developed that alternates frequent low- and high-magnitude floods, complemented by targeted sediment replenishment where necessary. This strategy ensures that restoration measures remain compatible with operational constraints while promoting floodplain functioning. By maintaining a disturbance regime closer to natural conditions, the approach aims to sustain biodiversity, reactivate dynamic sediment processes, and better balance ecological needs with hydropower requirements in regulated rivers and floodplains.