It runs approximately 83 miles (134 km) from the Sierra Nevada Range to the San Joaquin River if measured from the source of Rainier Creek, near Raymond Mountain in Yosemite National Park.

Headwaters: The Fresno River forms in the Oakhurst valley, near the western border of city limits.

The primary source of the Fresno River is Lewis Fork Creek, which gathers water far northeastward into the hills adjacent to Fish Camp.

Now a popular recreation spot, the lake covers a considerable area when full from late spring snow melts.

At Road 17, the natural riverbed has been modified to divert most flows into a manmade canal, which leads to the Eastside Bypass.

Once in the bypass, water can exit via a small channel at a diversion dam and continue west along the natural riverbed the rest of the way to the San Joaquin River.

Between the Eastside Bypass and the San Joaquin River, the riverbed (which is almost always dry) has been subject to straightening, but more or less follows its natural course.