They do not mix. Trout need gravel for spawning habitat.
Sand covers it up.
It's an age-old battle, especially here in Michigan where, ever since the logging era, there has been all too much sand and not enough gravel in the rivers. Now comes what many trout enthusiasts are hailing as the future of stream restoration - the Stream Sweeper. It can be taken into a river wherever there is access and floated downstream to sites that can't be reached with conventional heavy equipment. And it eliminates the need to truck sediments out to disposal areas. Using a pump and up to a half mile of six-inch pipe, the Sweeper sucks up a slurry of sand and water and moves it to upland sites.