One dirty fish spoils the entire pond

I once thought that lakes are singular water bodies. To my surprise, many lakes are sourced by other water bodies such as streams or rivers, and in cities which rely on lakes such as Bangalore, all the lakes are interconnected by a system of streams. These keep the water in the lakes balanced, an overflowing lake would transfer water downstream into another lake with a greater capacity.

Therefore we can say that in a lake valley, the water quality and content would be similar. If a lake upstream is polluted, a lake downstream would suffer similar consequences. This connectivity did not allow an overflow of water out of the lake into the surrounding area as an additional quantity of water was transferred to other lakes through the channels.

Let me take the example of the Varthur and Bellandur lakes. These lakes are the largest and the most polluted lakes in Bangalore, with very similar water quality. Notice how these lakes lie in the same valley system and are interconnected through various channels.