You are correct there. Normally most of the water comes down the Grose river into the lower Nepean. Much extra flow of the water during widespread heavy rains from the North comes down the Hawkesbury River and backs up the Nepean. The flow in the lower Nepean River often reverses direction. The Warragamba Dam is also above the upper Nepean inputs of Glenbrook and Erskine "Creeks" which also turn into raging flows during heavy rains.
