Casserly Creek is a 6.2-mile-long (10.0 km)[2] stream originating on Mount Madonna in the southern Santa Cruz Mountains in Santa Clara County and flows southwest into Santa Cruz County and ends in a large marsh, which drains an additional 0.6 miles (0.97 km) into College Lake and Salsipuedes Creek, which, in turns, flows to the Pajaro River, and thence to Monterey Bay and the Pacific Ocean.

[3] Casserly Creek has its origin just west of the peak of Mount Madonna in the southern Santa Cruz Mountains in Santa Clara County.

It flows southwest until it is joined from the left by Gaffey Creek in Banks Canyon just before crossing into Santa Cruz County.

[5] Just south of Interlaken it ends in a large marsh, which in turns drains into College Lake, the upper part of Salsipuedes Creek.,[6] which is in turn, tributary to the Pajaro River.

Casserly Creek is continuously used by steelhead trout (Oncorhynchus mykiss), and a 2007 report by Dr. Jerry Smith found multiple trout age classes from College Lake to the Banks Canyon/Gaffey Creek confluence.