course across Modoc County, past Canby and through the Modoc National Forest in the narrow Stonecoal Valley Gorge. It turns south to flow past Lookout and into northern Lassen County, past Bieber, to emerge into the ranching region of Big Valley. North of Little Valley it flows into northeast Shasta County and the Shasta National Forest. Then the river reaches Fall River Valley, where it is joined by the Fall River, which is fed by one of the largest freshwater spring systems in the United States. After passing through the town of Fall River Mills, the river drops over Pit River Falls, then enters the head of a long serpentine canyon that cuts through the southern Cascade Range. It then turns south to join the Sacramento River as the eastern arm of Shasta Lake reservoir, approximately north of Redding. Potem Creek joins the river at Potem Falls. Two major tributaries, Squaw Creek and the McCloud River, join the Pit from the north within the lake. The lower of the river forms the longest of the five arms of Shasta Lake, which is formed by Shasta Dam on the Sacramento downstream from the original confluence. Hydrology Fed by significant volcanic groundwater basins that produce some of the largest contiguous freshwater spring systems in the United States, the middle and lower reaches of the Pit River exhibit a strong year-round flow, in contrast to the highly seasonal nature of most northern California rivers. Before Shasta Dam was built, the Pit contributed as much as 85 percent of the Sacramento River's dry-season flow as measured at Red Bluff, nearly downstream of their confluence – making the river an important resource for irrigation, and later, hydroelectricity. The upper reaches of the Pit above Fall River Mills are a snow-fed high desert stream with a much more seasonal hydrograph. The lowermost part (Shasta Lake portion) of the Pit River system receives heavy winter rainfall, which mainly contributes to streamflow between November and April. Nevertheless, summer low water flows rarely drop below . 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