Where rivers flow into basins created by geological faults, the water accumulates into immense lakes. Lake Prespa in the former Yugoslavia is 20 miles long. As the rivers that feed these lakes lose their impetus they drop sediment, making lakes like this very fertile. The animal inhabitants are no longer harassed by a perpetual current nor hemmed in by a narrow bank, so they are free to proliferate. Fish swarm in Lake Prespa as do fish-eating birds like cormorants and pelicans. The most fertile parts of the lake may be the margins where, thanks to the sun, algae flourish, snakes thrive, small invertebrates proliferate and there is food for even the least agile of hunters such as turtles. Just a few miles away in Lake Ohrid, there is a distinctive red trout that only lives in that lake. Because of their isolation, communities of fish become very inbred and the fish evolve into new species. A similar thing has happened to the shrimps here as well as several species of water snail.