Placement of spawning gravel in the river

Description of functioning:

The river regulations often change the natural flow regime and the sediment connectivity, by this introducing changes to the substrate composition both in the bypass section and downstream the outlet of the hydropower plant. In particular, the reduction in magnitude, frequency and duration of floods impact the substrate composition, typically leading to fine materials clogging the substrate and possibly creating an armoured layer. An armoured layer will inhibit the spawning of fish species laying their eggs in the substrate, potentially reducing the number of eggs deposited in the substrate, increasing the predation and possibly also reducing the survival of eggs, e.g. due to low oxygen levels in the hyporheic zone. As such, the areas supporting spawning can be reduced due to regulation and hence represent a limiting factor ('bottleneck') for the fish population. The grain size of distribution of the spawning gravel to be placed in the river must be such it supports the species of concern. It must not consist of substrate the spawning fish is able to dig and lay their egg in, and not so fine that clogging will occur. The shape of the stones should as the natural conditions in the river, and sharp-edged stones from blasting, often available close to a hydropower project, should only be used if considered appropriate for the species of concern. If the gravel is not sufficient 'clean', it should be washed prior to deposited in the river, in order to avoid particle pollution and possibly increased clogging downstream. Before placement of spawning gravel in the river is made, the hydraulic conditions where the spawning gravel is placed must be investigated. The gravel must be located in a part of the river that does not dry out during low flow conditions, in areas with sufficient through-flow of fresh, oxygen-rich water to the eggs, and in areas that are not exposed to out-wash/flushing during high flow events. This measure has been implemented in a number of rivers in Norway. It is a fairly cheap measure to introduce, it stimulates the natural population and seems to achieve very good results in all rivers it has been used.