A road can make landslides and erosion more likely and more severe. And a road near a stream creates an added threat to the fish that live there. Building a road along the hill is like adding a rain gutter to your roof. When rain falls on the roof, it flows down to the gutter where it is concentrated into a stream that shoots quickly down a spout with a lot of force. Now imagine the hill with a road and a stream nearby.
Rain is concentrated by the road.
It picks up soil, sand and sediments. When it hits the water, this muddy runoff can bury and kill fish eggs. To make matters worse, fish often lay their eggs during the rainy season. Since Pacific salmon only lay eggs once in their lifetimes, the loss of eggs can have a major impact on fish populations. You can build roads far away from streams and sensitive areas.