The former has worked with the University of California Davis to study the trout and identified two distinct sub-populations, one adapted to the colder winter flows of Bear Creek and one adapted to the more constant temperature spring-fed waters of Spring Creek and the Fall River mainstem.

[4] The upper river has historically been characterized by abundant aquatic macrophytes, including extensive meadows of horned pondweed (Zannichellia palustris), however these have dramatically declined due to excessive sediment deposition due to fires in the watershed and channelization of a tributary stream.

[7] The Fall River watershed drains a 612 square miles (1,590 km2) area that originates from Thousand Springs, and is largely spring-fed, with Bear Creek providing the only significant precipitation-related surface flow to the river.

Bear Creek rises at a source elevation over 4,803 feet (1,464 m) and flows east to join Fall River at Thousand Springs at an elevation of 3,323 feet (1,013 m).

Fall River is a moderate sized, slow moving, meandering meadow stream with a mean gradient of less than 1 ft./mile.