Not all chemicals and pollutants are water soluble, and those that aren't are called suspended matter. The tiny particles of matter stay in the water and eventually fall to the bottom, forming a layer of silt on the floor of the lake or river. This is harmful to wildlife and causes long term problems due to an imbalance in the natural infrastructure of the water.
In addition to the problems caused by the suspended matter, the problem caused by pollution due to suspended matter is compounded by dead fish and wildlife decomposing in the water.