Rivers and lakes are fresh surface waters, which also include streams and other water bodies. More than 250 species of algae and 500 species of fish, with significant number of birds and mammals, depend on freshwater habitats of European inland waters. River ecosystems of moderate climate zones are richest in species due to natural connection and great diversity of habitats, while lakes and other stagnant waters positively affect the quality and quantity of water. Rivers themselves transport water and sediment downstream, which prevents flooding and increases the abundance of nutrients in the water.