The Humber River gurgles up from cool springs near the base of the Niagara Escarpment in Dufferin and Simcoe Counties and the Town of Caledon, just north of Mono Mills. The river then meanders for 126 km across the Oak Ridges Moraine and the Peel Plain to its mouth at Lake Ontario.

Numerous small tributaries coalesce into creeks in the rolling hills of the Oak Ridges Moraine uplands to become the Main Branch of the Humber. The moraine is an enormous deposit of mixed sand and gravels left behind by the glacier as it retreated from this area over 12,000 years ago. Ever since the glacial melt back, water courses have been cutting deep meandering valleys and gullies into its rolling hills, contributing significantly to the natural beauty of the area.

The moraine, with its scenic hills and conspicuous wooded valleys, is especially valued as the natural aquifer that sustains Lake Ontario. It is the primary source of drinking water for the millions of people who live in the watershed.

As the Humber River spilled out of the moraine rushing toward the Peel Plain, it provided the water power which fostered development of historic mills and the establishment of villages such as Palgrave and Bolton. The river descends more gently across the Peel Clay Plain to Lake Ontario, passing through Kleinburg before joining the East Branch of the Humber at Woodbridge.