Bed load and suspended solids in watercourses

Sediment is an important component of ecologically intact rivers and streams. However, the sediment budget is disrupted in around one-third of watercourses. The FOEN monitors the transport of sediment by watercourses across Switzerland.

Sediment is transported in a body of water in two ways: as bed load or as suspended solids. The bed load and suspended solids budget is important for functioning ecosystems in streams and rivers. As the FOEN's studies show, suspended solids and sediment in Swiss watercourses are loaded with organic micropollutants. The natural bed load budget is adversely affected by the impacts of river control structures, bed load traps and hydropower plants. A disrupted bed load budget does not only harm plants and animals, it also adversely affects groundwater. Bed loads and sediment work like filters on the channel bed between the water in the river and the water on the floor.

Overview of the bed load budget in Switzerland

In connection with strategic planning to renature bodies of water, cantons have had to perform surveys, which provide for first time ever an overall picture of the bed load budget in Switzerland as a whole: Around one-third of the estimated 10,000 kilometres of studied bodies of water do not meet the requirements and need to be remediated. A total of approximately 500 installations are concerned by the required remediation measures. One-third of these are power plants, while the rest are other installations such as bed load traps.

A study commissioned by the FOEN has investigated suspended solids and the proportions of fine sediment, i.e. particles that are smaller than 63 micrometres, in the Rhine, Thur, Aare, Reuss, Limmat, Birs, Rhone, Ticino and Inn Rivers. The study revealed that both suspended solids and sediment are loaded with organic micropollutants. However, the measured concentrations of most metals have decreased over time.

Consequences of reduced bed load transport

The bed load budget shapes the morphology, the substrate, the dynamics and thus the habitat of a stream or river. It has a huge influence on the ecology of a watercourse: If the bed load transport is reduced, the bed erodes, causing unnatural erosion. There is also a risk of shores washing away, flood protection structures being destroyed and bridge pillars eroding. Surrounding land is affected by the sinking groundwater table. Channel beds are also covered with fine matter, which limits spawning opportunities for fish.

Changes in the natural transport of stones, gravel, sand and suspended solids also cause economic damage. As a result, deposits in river channels can lead to flooding problems because watercourses with reduced flow capacity rise over the banks faster, which can sometimes cause a great deal of damage. Yet, too much bed load upstream from installations is also a problem. It causes water reservoirs to silt up, shrinking their holding capacity.

Geschiebe- und Schwebstoffproblematik in Schweizer Fliessgewässern. (PDF, 2 MB, 28.08.2007)Schälchli, Abegg + Hunzinger, 2005. Studie im Auftrag des BAFU.

Monitoring network and monitoring

The FOEN operates a nationwide monitoring network for bed load sediment transported in Swiss watercourses. The causes of these adverse effects vary depending on the region: In fact, hydropower plants along large rivers in the Central Plateau have the largest influence, while the bed load budget in smaller rivers in the Central Plateau is being disrupted by bed load traps. In the Alpine region, the causes are gravel extractions, power plants and river control structures and corrections.

Bed load is transported on the floor of streams and rivers. It is composed of sand, coarser bed load and even blocks. Suspended solids are fine particles of clay and mud, and also take the form of sand in rivers with larger slopes and mountain streams. The FOEN monitors the transport of sediment in Swiss watercourses. This includes measuring concentrations of suspended solids and turbidity and recording the quantities of bed load matter in bed load traps. In previous years, river deltas were also surveyed.

The data from the surveys can be obtained from the FOEN.

Last modification 11.04.2022

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