Dams have significant impacts on human communities and ecosystems. The flooding upstream of dams can destroy existing communities, archeological sites (as in the Aswan Dam in Egypt or Glen Canyon Dam on the Colorado River), and wildlife habit. Dams are barriers to fish migration (especially of salmon) and often have serious effects on fish and fishing. Dams can reduce downstream water flows for the entire length of a river, affecting communities for thousands of miles downstream by reducing available drinking and irrigation water. Dams can also silt up, limiting their lifespan unless they are maintained by dredging, and sometimes dams collapse causing massive flooding downstream that would not have happened had the dam not been built.