Change of sediment, amount and particle sizes, during salmon spawning in an artificial channel in British Colombia, Canada

Abstract

Most research examines the spawning habitat and the effect of sediment at the spawning locations, while the possible effect of spawning activity on the location is uncertain. The decision to spawn at a certain location depends mainly on the water depth, water velocity, and the amount of gravel-stored fine sediment, since the survival of the eggs depends on a constant supply of oxygen. Small particles, such as clay and silt, may block the water flow in the redd and cause egg mortality. Other research examines the effect of the dead carcasses after spawning on the river in nutrient concentrations and changes in sediment composition.

Nevertheless, the activity of salmon during spawning affects the location as well, because it involves: 1. Digging a nest (redd) in the gravel bed of a river; 2. Cleaning the gravel from fine sediment; 3. Depositing the eggs; 4. Covering the eggs with gravel. These actions change the suspended sediment concentration and stored fine sediment concentration in the spawning habitat. This effect could be large enough to result in significant differences. The main aim of this research is to study the change in fine sediment during salmon spawning in an artificial channel: Horsefly Spawning Channel, near the town Horsefly (British Colombia, Canada).

This study results in a minor effect of spawning on the sediment concentration in the channel and a major effect of sediment input from the river. There is even more sediment input during high discharge and a conservative linear relation is the result. This major effect was not expected, since the settling ponds upstream of the spawning locations and sampling points should reduce the sediment input. Nevertheless, the water velocity in the settling ponds is too high for all the sediment to settle down. This effect is even higher when the discharge increases and the river water also contains more sediment due to this high discharge and possible rainfall events. The increase of clay and silt particles during high discharge events is caused by the higher velocity in the settling ponds, when less small particles settle down.

Even so, this study results in an effect of salmon spawning. During the low discharge event, the concentration suspended sediment is high and is mainly caused by the high spawning activity. Furthermore there is a slight increase of suspended sediment during the days of most intensive spawning activity. The silt content decreases during the days of most intensive spawning activity. Clay content increases severe by an increase of spawning activity. This proves the effect of spawning, because small particles are longer retrained in the water column (for example after resuspension). Whereas the larger silt and sand particles are deposited within a short distance from the redd and the major part of these large particles is stored again in the channel.