While conducting surveys for irrigation projects in the early 1900s, the U.S. Reclamation Service (now Bureau of Reclamation) noted that the spring-fed Fall River alone contributed a year-round flow of about , from an aquifer fed in part by Mount Shasta snowmelt. Much of this water rises at what is often called Thousand Springs a few miles above Fall River Mills, west of Ahjumawi Lava Springs State Park. Hat Creek and Burney Creek, spring-fed from the Lassen Peak area, supplied a further to the Pit River. The aquifers in the Pit River basin may hold as much as in storage and are consistently replenished by winter precipitation seeping through the watershed's porous volcanic rocks and soils. The water typically emerges at points of lower elevation where the surface layers encounter harder metamorphic and sedimentary rock.