Köhler et al. (2025) Dense stands of aquatic plants retain water in lowland rivers and in adjacent floodplain aquifers

Identification

• Journal: Journal of Hydrology
• Year: 2025
• Date: 2025-12-29
• Authors: Jan Köhler, J. A. Lewandowski
• DOI: 10.1016/j.jhydrol.2025.134882

Research Groups

• Leibniz Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany
• Geography Department, Humboldt University, Berlin, Germany

Short Summary

Dense stands of aquatic plants in a lowland river effectively compensate for declining discharge by elevating water levels, significantly increasing water storage in both the river channel and the adjacent floodplain aquifer, with weed cutting offering only a temporary reduction in this beneficial water retention.

Objective

• (H1) aquatic vegetation compensates for declining water levels in a drier climate;
• (H2) macrophytes retain water not only in the river channel but also in the adjacent aquifer;
• (H3) weed cutting lowers the water level and reduces water storage capacity for few weeks and short river sections.

Study Configuration

• Spatial Scale: A 32.4 km long section of the River Spree, southeast of Berlin, Germany. The adjacent floodplain has an average width of approximately 1000 m. Hydrological measurements were taken at 14 water level gauges and 2 discharge sites, with groundwater monitoring wells along a transect perpendicular to the river.
• Temporal Scale: Long-term hydrological data (discharge and water level) from 1965 to 2022. Macrophyte biomass data from 1988, with detailed mapping since 2001. Groundwater level records from 2006 (hourly intervals). Specific analyses for water retention focused on the period 2011–2021.

Methodology and Data

• Models used:
  • Impounding effect calculation: Determined as the deviation of measured water levels from a theoretical water level, which was derived from winter (macrophyte-free) gauge-discharge relationships.
  • Water volume calculation: Based on cross-sectional profiles of the river channel and daily water level data.
  • Aquifer storage estimation: Calculated as the product of the water level increase, the estimated area of the influenced floodplain aquifer, and a mean water content of 20 vol-% for the saturated aquifer sediment.
• Data sources:
  • Long-term discharge and water level data from various German authorities (Waterways and Shipping Authority Spree-Havel, Environmental Agency of Brandenburg, Water and Landscape Management “Lower Spree”).
  • Macrophyte biomass: Estimated through visual inspection, Plant Volume Infested (PVI) method, and harvesting dry weight at a standard transect, supplemented by coverage estimates in mowed sections.
  • Groundwater level records: Automatic measurements at 1-hour intervals from a transect of wells equipped with pressure probes (Aquitronic data loggers).
  • River morphometry: Cross-sectional profiles of the river channel measured every 200 m.

Main Results

• In recent summers (2011–2021), aquatic macrophytes caused a water level increase of 64 ± 14 cm at the upstream end of the river section, effectively maintaining a constant mean water level despite a 47% decline in discharge.
• The impounding effect of aquatic vegetation retained an additional 0.20 ± 0.11 million cubic meters of water in the river channel, representing a 19.4% increase over the theoretical volume without plants.
• This effect also led to an additional storage of 1.47 ± 0.67 million cubic meters of groundwater in the adjacent floodplain aquifer, an increase of up to 143% compared to the original river channel storage without vegetation.
• River water level fluctuations propagated rapidly into the floodplain aquifer at a velocity of 1550 m per day, with groundwater levels closely mirroring river levels.
• Mowing in July reduced the impounding effect for at least 4–6 weeks, decreasing the retained water volume in mowed river sections by 68–80% and the additional groundwater volume by 40–47%. Mowing in September showed no additional reduction beyond natural plant decay.
• Aquatic vegetation reduced the mean flow velocity in the entire river section by 62% during June-August (2011–2021).

Contributions

• This study is the first to quantitatively assess and demonstrate the significant volume of water retained by dense aquatic vegetation in both the river channel and its adjacent floodplain aquifer.
• It provides strong evidence that aquatic vegetation can act as a natural buffer against declining water levels in lowland rivers, effectively compensating for substantial reductions in summer discharge, thereby offering a crucial adaptation strategy in a drier climate.
• The research challenges conventional weed management practices by highlighting the substantial benefits of aquatic plants for water retention, suggesting these benefits may outweigh perceived flood risks, especially in drought-prone regions.
• The paper validates a robust, long-term method for estimating the integrated impounding effect of aquatic vegetation across extended river stretches using routine hydrological measurements.

Funding

• German Federal Ministry of Education and Research (project code: 02WGR005) as part of the Water Challenges for a Changing World Joint Programme Initiative (Water JPI).

Citation

@article{Köhler2025Dense,
  author = {Köhler, Jan and Lewandowski, J. A.},
  title = {Dense stands of aquatic plants retain water in lowland rivers and in adjacent floodplain aquifers},
  journal = {Journal of Hydrology},
  year = {2025},
  doi = {10.1016/j.jhydrol.2025.134882},
  url = {https://doi.org/10.1016/j.jhydrol.2025.134882}
}