The City is also identifying how to avoid, minimize, or mitigate activities that have a negative impact on these fish. Development activities can pollute water, degrade instream and riparian habitat, and alter the natural flow of rivers and streams. Erosion Erosion can put excessive amounts of sediment into rivers and streams, and can be lethal to salmon and trout. Both species need gravel and rocks to spawn and rear young. Erosion caused by construction introduces fine sediments that clog the spaces between rocks and gravel in streams, buries the eggs salmon and trout lay in these spaces, and prevents flowing water and oxygen from reaching the eggs and newly hatched fish.
Sedimentation can also fill in pools, which are an important part of fish habitat.
Salmon and trout use pools for rearing and spawning, as resting areas during migration, and as a refuge to avoid temperature and flow extremes. Sediments in water can damage gills and decrease visibility, which can hamper the fish s ability to find food. Sediments also can carry and store toxic pollutants and nutrients that can poison habitat. Portland is developing a new system to track and respond to erosion problems. The City is rewriting its erosion control regulations and design manual to improve construction site erosion control and stormwater management.