A whole chain of events is set in motion throughout the aquatic ecosystem. Since most biochemical processes speed up as the temperature increases, warmer water can push the system into high gear. Decomposers work and use oxygen at a faster rate. To compound the problem, warm water holds less oxygen than cold water. Some species can't survive in warm water, either due to lower oxygen levels, or because they are sensitive to heat.
Less water is able to infiltrate the soil to replenish groundwater, so lower summer flows occur.
Asphalt surfaces also heat the water entering the river. Pollutants such as oils, trash and even dirt are washed right into the river instead of being filtered through the soil. Erosion Now let's add some sediment from eroding crop land, construction sites, logging areas, and bank erosion. Sediment buries the homes and clogs the gills of many aquatic organisms, and catches and holds heat from the sun, causing warmer water temperatures. Often, nutrients and other pollutants, including heavy metals and toxics, attach to sediment, adding to the damage it causes when washed into streams.