Salmonids need stable uncompacted substrate to spawn in salmon and large trout prefer stones ranging from the size of a golf ball to the size of a tennis ball. Smaller trout prefer stones the size of a  pea to tennis ball size. If the substrate becomes silted the eggs or young alevins/fry will suffocate.
Restoration of Spawning gravels (redds). If  the redds are silted up, simply raking or flushing them out is only a short term method as the problem will lie further upstream. In creating a spawning habitat some care has to be taken to help eradicate the siltation during high water flows.