The Gualala Basin drains an area of 298 square miles along the coast of southern Mendocino and northern Sonoma counties. The river enters the Pacific Ocean near the town of Gualala, approximately 115 miles north of San Francisco and seventeen miles south of Point Arena. The Gualala Basin is about thirty-two miles long on a northwest – southeast orientation, and extends inland about fourteen miles. Elevations vary from sea level to 2,602 feet at Gube Mountain. The most mountainous terrain is in the northern and eastern parts of the watershed.
The Gualala Basin has a Mediterranean climate and is influenced by fog near the coast with seasonal temperatures ranging from 40 to 60° F, with the interior areas of the watershed ranging from below freezing to over 90° F seasonally. Rainfall also varies by location within the watershed with an average of 33 inches falling near the town of Gualala, and totals reaching over 63 inches in some areas of the interior.
A long history of movement along the San Andreas and Tombs Creek faults has been a dominant force in shaping the Gualala Basin. These faults transect the basin along northwest-southeast oriented lines. The Tombs Creek Fault separates highly unstable mélange on the east from relatively stable terrain on the west. The South Fork Gualala and the Little North Fork of the Gualala River flow within a linear valley formed by the San Andreas Fault. Bedrock underlying much of the basin has been tectonically broken and sheared making it relatively weak, easily weathered, and inherently susceptible to landsliding and erosion. <
Prior to European settlement, coniferous forest occupied approximately two thirds of the Gualala Basin. Dense old growth redwood forests dominated the northwestern portion of the basin, particularly the alluvial North Fork Subbasin. Old growth redwood forest also lined the long and narrow South Fork Gualala valley. Douglas fir predominated in central and mid-slope locations more distant from the coast. In the inland, eastern portion of the Gualala Basin, the natural distribution of Douglas fir becomes increasingly fragmented. Here, the long summer drought limits Douglas fir to north facing slopes. Oak-woodland predominates on higher, inland terrain that is beyond the foggy coastal marine influence. Additionally, large areas of prairie grassland occupy the driest sites along the higher slopes and ridges. These grasslands occupy larger continuous areas on the highest and most eastern areas of the basin.
The total Gualala Basin resident population for the year 2000 census was estimated to be about 2,700 people with the majority residing in or near the town of Gualala and less in and around the town of Annapolis. Over 99 percent of the basin is held as private property. Compared to other north coast watersheds, the basin has one of the longest records of timber harvest as a primary land use.
The current salmonid resources of the Gualala Basin include coho salmon and steelhead trout and above barriers to anadromy, resident populations of rainbow trout exist. Chinook salmon juveniles were caught prior to 1945. The Gualala Basin had been extensively stocked with both juvenile coho salmon (1969-1998) and steelhead trout(1971-2001).