Rain falls on the higher ground and flows through over ground and under ground streams and rivers to the lake, down the falls, into another lake into another river and eventually out to sea.
In the meantime heat from the sun causes water to evaporate from the earth's surface and from lakes and rivers and seas to form clouds. The clouds grow bigger and are blown in the wind. Once heavy enough they burst and the watre vapour falls as rain. The rain runs into the streams and rivers and off down those falls again.