Bump and Grind in the Gualala

by the GRWC Student Interns with help from Wayne Haydon, CGS…

Lidar image of San Andreas Fault Zone

Lidar image of the San Andreas Fault Zone along the South Fork and Little North Fork rivers in the Gualala River Watershed. Lidar coverage courtesy of USGS and Gualala Redwoods, Inc.

The Gualala River watershed and surrounding areas is an ever-changing, very uneven landscape. This region has been evolving for the last 30 million years (and most so in the last 5 million years). There are four main reasons for the local landscapes unevenness and diversity. The first was the passage of the Mendocino Triple Junction of tectonic plates; that is, where the north-moving Pacific Plate (east of the South Fork and the Little North Fork of the Gualala River and out in the Pacific Ocean) ground past the west-moving North American Plate. As the plates continue to grind past each other, it causes the earths’ crust to be uplifted, shortened and “scrunched up” in places, pulled and torn apart in other areas, with one plate sometimes being forced underneath another plate. Northern Coastal California is a fault riddled area and has been consequently “crumpled” by the grinding together of these three plates creating a constantly changing geography. The Mendocino Triple Junction is today in the vicinity of Petrolia.

The second reason for the unevenness occurred during the last ice age. As glaciers advanced, more and more of the earth’s water was locked up as ice. Sea levels around the world dropped, creating deep river valleys graded to the new lower sea level. When the ice age ended, water began to fill these valleys again, the river became graded to the latest higher sea level, and these valleys filled with the sediment present today.

Thirdly, the present day landscape of the Gualala is influenced by the continuing legacy of the Mendocino Triple Junction. This passage created the still active San Andreas Fault and the associated earthquakes and other now inactive faults. The area is heavily influenced by these faults, most notably the still active San Andrea’s and the now inactive Tombs Creek Fault System.

Gualala River and the San Andreas Fault

Valley of the South Fork Gualala River along the San Andreas Fault Zone


The South Fork and the Little North Fork of the Gualala River flow directly through the San Andrea’s fault valley. Even the inactive faults influence stream direction because of how streams will follow the softer fault weakened rock, due to their ability to carve them out. This is a very prominent contributing factor to the many hills, ridges and valleys of the region.

Additionally, the present day landscape of the Gualala River continues to change through the processes of erosion and mass landslides. The landsliding results from the uplifting of weak rocks to high relief over relatively short geologically time periods, abundant rainfall and frequent large magnitude earthquakes that shake the hill slopes. Landslides and erosion continue to be the most prominent source of change in the watershed’s geology.