KRIS Shasta River Tour

Geology: Introduction

The Shasta River drains two geologic provinces: the Cascade Province to the east and the Klamath Province to the west. The valley is nearly oval in shape, and is surrounded by mountains. The eastern part of the valley is underlain with basaltic lava which erupted from the flanks of Mt. Shasta and flowed across the valley floor in recent geologic history. Covering more than 50 square miles, this so-called Plutos Caves basalt contains underground tubes created by trapped air as the flowing lava cooled. The Shasta River winds across this young, basaltic formation.

Mount Shasta, part of the Cascade Range, looms large above the Shasta River landscape.

Mount Shasta dominates the landscape almost everywhere in the Shasta River watershed. At 14,162 feet high, the mountain looms far above everything else. Several glaciers exist on the mountain, and glaciers descended the slopes of the mountain to an altitude of 2,800 feet during the Pleistocene epoch . Glacial moraines deposited during that time can be seen in the southern part of the valley. Today the glaciers continue to deposit materials on the lower northwest flank of Mt. Shasta.

A geologist recently discovered that the hillocks covering much of the central valley floor were deposited in a huge debris-avalanche. A mountain similar in size to Mount Shasta actually liquefied when an eruption caused its glaciers to melt. The slurry of water and rocks moved out onto the valley floor, forming rounded hills interspersed with depressions. The depressions fill with water in the winter, and provide wetland habitat. These formations can be seen along Interstate 5 between Weed and Grenada.

Mt. Shasta is on the left. The small, cone-shaped mountain to the right of Mt. Shasta is approximately where the mountain that liquefied once stood. Just in front of these mountains are the hillocks that were formed from the material that once made up the mountain. The Klamath Mountains are on the right.

From the KRIS bibliography

For more information about the geology of the Shasta River:

Gigantic Debris Avalanche of Pleistocene Age from Ancestral Mount Shasta Volcano, California, and Debris-Avalanche Hazard Zonation. U.S. Geological Survey Bulletin 1861. 1988. A portion of this report is presented for more in-depth information about the debris avalanche and the geology of the Shasta Valley.

Geology Section from Shasta/Klamath Rivers Water Quality Study. Department of Water Resources, Northwest District. February 1986. For a general description of Shasta River and portions of the Klamath River watersheds.

Introduction

Cascade and Klamath Ranges

Map of River and Mountain Ranges

Guided Tour Introduction

Fisheries

Water Quality

Hydrology

Economy

Restoration