Forests retain enormous amounts of rain water - about 800 tons per acre

From The Johnstown Flood by David McCullough. Page 65.

Forests not only retain enormous amounts of water in the soil (about 800 tons per acre), but in mountainous country especially, they hold the soil itself, and in winter they hold snow. Where the forests were destroyed, spring thaws and summer thunderstorms would send torrents racing down the mountainsides; and each year the torrents grew worse as the water itself tore away at the soil and what little ground cover there was left. Then, in the valley, where the water was being dumped ever more suddenly, the size of rivers which had to carry it all was being steadily whittled away at by industry and the growing population. So there was always a little less river to handle more runoff, and flash floods were the inevitable result.