On an early fall trip to northern California, we had the pleasure of visiting the streamcourse of the mighty Sacramento River. Draining the high altitude lava fields of northeastern California at the southern edge of the Cascade range, the Sacramento originates with the Pit and McCloud Rivers and flows about 350 miles to the San Francisco Bay delta. Even though it traverses rich agricultural lands, it holds clear water most of the year, with transparencies measured in meters over at least 75% of its course. We never see water clarity of that kind in the large rivers of the midwest and eastern US, so it is reasonable to ask what are the differences in the origins of silt loads and turbidity between these systems.
Our analysis boils down to only three critical environmental features: flow rate, bedrock characteristics, and soil overburden.
Episodic flows occur only with winter rains and spring snowmelt. Most of the flowage comes from the windward (western) slope of the Sierras and Cascades as the Coast Range provides a rain shadow over the valley. Flow rates in the upper river are further moderated by temporary holding of headwaters in Shasta Lake for hydroelectric power generation and release later in the season.
Bedrock in the upper reaches is basalt or insoluble lava or other igneous rock, leading to very low mineral concentrations in runoff and very poor biological productivity. High gradient tributaries flowing in from the Sierras also drain insoluble granitic bedrock. Percolation is rapid from surface into groundwater aquifers through fractures and lava flow boundaries, including numerous lava tubes.
McArthur-Burney Falls on the Fall River, we see significant flows emerging from the face of the lava wall over which the bulk of the water falls. The subterranean channels whose termini we can see at the falls form a groundwater network of great extent under the lava fields surrounding Mount Shasta.
Soil overburden is generally thin in upper reaches and held in place by evergreen forest and perennial shrubs. High altitude meadows are generally unsuitable for tillage but may support low intensity grazing on perennial grasses. Soil integrity in mountainous areas has been disturbed only by fire, clear-cut deforestation, recently being replaced by more selective logging practices, and hydraulic mining scars left over from the Nineteenth Century.
In the arid high elevations, and in the foothills below the Sierra-Cascade crest down to the Sacramento valley floor, the dominant vegetation is a drought-tolerant assemblage of oaks and evergreens interspersed in vast expanses of chaparral. Significant species of shrubs include rabbit brush, manzanita and buckthorn so entangled as to make foot passage nearly impossible in river canyons. The density of this vegetation in the Mill Creek and Deer Creek canyons is credited with hiding the existence of Ishi, last of the California Yahi Indians, for fully 40 years in the closing decades of the Nineteenth Century.
From Redding down to the Bay Area delta, the river flows through some of the richest agricultural soils in the world. The gravels underlying modern valley soils are glacial outwash deposits no more recent than the Tioga glacial recession of about 15,000 years ago. The soils themselves are deposits of glacial fines accruing organic matter annually from vegetation cycling. Agriculture today is mostly stone fruit and nut tree orchards with replacement schedules of decades. In the lower valley, intensive rice culture with controlled flow paddies dominates annual plantings. Further diversion of Sacramento River flows for irrigation and domestic water supplies severely limit late-season fish migration opportunities both upstream and down.
The result of all this is that high sediment loads in the Sacramento river occur only during winter flooding when flows are sufficient to scour riparian surfaces and re-suspend sediments previously deposited in the river valley. For trout and salmon, headwater habitats remain viable but the great mainstem migrations of salmon and steelhead are now gone or mere shadows of those a century ago. Severe water quality problems for over half a century in the San Francisco Bay area, combined with inadequate upstream fish passage and insufficient flows to deliver smolts to the marine environment, make the prospects grim for the remaining threatened and endangered salmonids.
One exception is the near-heroic attempt to protect and sustain the Butte Creek spring run chinook salmon. Only time will tell if the recent management efforts and habitat restoration programs in the Sacramento River basin will be sufficient to rebuild and restore some elements of the once-great salmon and steelhead runs in the north central valley.
Prominent trout streams in the Sacramento River watershed: McCloud R., Fall R., Hat Cr., Pit R., Deer Cr., Butte Cr., Feather R., Yuba R., American R., Sacramento R.