by Reid Bryson
May 31, 2002

In the past half-century few inhabited areas of the United States have managed to support a steady population and retain a respectable degree of autonomy. The Mattole Watershed is one of those few places. By virtue of its inhabitants’ physical and ideological distances from the rest of the country, a type of local perspective has taken root here, while in most modern communities anything local is often diluted or lost altogether. For many residents and other concerned citizens, the sense of isolation here is a treasured quality. However, isolation guarantees neither serenity nor purity. There is perhaps no better illustration of this truth than the issue of sedimentation in the Mattole River and its tributaries.

The erosion of land and subsequent sedimentation of associated watercourses is an entirely natural process. Rivers, creeks and streams around the world continually redirect their course. This is accomplished not by a moving of the watercourse itself but by a moving the land around it, particle by particle, from one place to another.

And in this age of chic environmentalism, where all things natural are wholesome by default, such an assertion might be the end of a discussion. But the Mattole is not the place for chic.

Plenty of substances and processes are both natural and detrimental to living organisms. In this category one finds the poisons produced by some animals to aid in the capture of their prey. Consider also the variety of natural disasters that adversely affect life on all parts of the planet. Would these examples find a place in the definitions of words such as wholesome or good? Gravity seems well and good until you find yourself with no floor.

Here we realize that the word “natural” and the word “good” are separated by a void that is large enough to encompass all things manmade. A more suitable partner for “natural” is “balance”.

Balance in relation to sedimentation of the Mattole has always been a tenuous arrangement. In a former configuration, approximately 145 million years ago, the site of the watershed was a seafloor where deposits of sedimentary, igneous and metamorphic rock began developing. This material makes up the watershed’s present geological foundation. Known as the Franciscan Formation, it is an exceptionally unstable configuration of sandstone, chert, shale and serpentine bonded together by clay. As these components assembled, the seafloor began to rise, and as recently as 15 million years ago sections of Franciscan Formation began to form the land that would become the King Range. That movement continues today as a result of the Mendocino Triple Junction. Some local rates of uplift are 1.3 feet per 1,000 years at Honeydew, 3.2 feet per 1,000 years at Whale Gulch and 10 feet per 1,000 years at Cape Mendocino.

It is important to remember that the uplifting process is not a gradual one. Pressure building along one of the three major axes (the Coastal Subduction Zone, the Mendocino Fracture Zone and the San Andreas Fault) is released in the earthquakes that frequent this place. These events do more than simply move the ground. They may also dislodge or injure the root structure of trees that provide soil retention on steep slopes and affect groundwater. Any equilibrium of sedimentation within the Mattole watershed must therefore find a niche in the unpredictable pattern of earthquakes in this land made of an unstable conglomerate.

As if that weren’t enough of an obstacle, precipitation also plays a role in determining rates of sedimentation, and is of no small consequence in the Mattole. Parts of our watershed near Honeydew typically receive over 100 inches of rain per year. Petrolia and Whitethorn receive, on average, 50 and 80 inches of rain per year, respectively. Some residents have even recorded annual rain totals of over 200 inches.

These conditions further intensify the tendency for erosion in the watershed. Clay, and its profusion throughout the watershed, is a special concern linked to precipitation because of the way it reacts under saturated conditions. Many residents are familiar with Blue Slide Creek and the continuous delivery of sediment to the Mattole River as a result of the landslide there. The blue coloration of the exposed ground at this and many other landslides in the Mattole watershed is an indicator of the high clay content of the soil at these sites. Clay particles are relatively small and generally flat, so that when arranged together, small spaces are formed where water may collect. However, when conditions for saturation exist, the additional weight from water can be enough to break the friction bonds holding a mass of soil together.

With all of these variables, it is clear why the Mattole watershed in particular is predisposed to extreme fluctuations in the balance of sedimentation. These are certainly not characteristics of a quiescent environment.

One aspect worth noting about precipitation is that the rainfall totals for the watershed are directly proportional to the river’s flow. Though this may seem obvious enough, the implication is that higher flows act as the primary transport for the eventual elimination of excess sediments from a river system. Indeed this has been the primary “natural” compensation for other factors contributing increased sediment levels to the Mattole.
Suddenly the system doesn’t seem so unbalanced. The goal of equalized sediment input and output appears attainable.

However, there remains another substantial source of sediment delivery to the Mattole River. This source is found within the influence of one species. And as such, is a clear example of impurity introduced into the Mattole watershed. The effects of human inhabitation have been responsible for substantial if not major portions of the sediment delivery that has occurred in the last 50 years. Much of this is attributed to the rapid increase of timber harvesting that took place for after World War II and lasted until 1975. Mechanized timber removal is associated with acute delivery of topsoil to the Mattole and its tributaries. These sources are slowly alleviated by the passage of time and reestablishment of vegetation on a logged parcel.

Road networks, as part of timber harvesting or not, are another common source of human initiated sediment delivery. Most of the roads in the watershed are not paved and as a result provide a chronic source of sediment via surface runoff and channeled drainage ditches. Roads are not limited to these effects, though. Particular concern is focused on the stream crossings where the potential exists for severe erosion of the roadbed. This often happens as a result of a blocked culvert at the crossing site. If debris is able to divert water from draining through the culvert, the stream’s flow will either be picked up by a drainage ditch or cause a road failure by swiftly eroding a significant portion of the roadbed. Either of these two events will result in more sediment delivery to nearby drainages.

Another potential road failure occurs in the form of landslides. Whether built along a ridgeline or across a steep slope, roads can compromise the structure of a larger area by establishing a zone of inconsistency in the landscape. This zone essentially provides an additional element of weakness to already unstable soils.

But the history of human habitation here begins well before the 1950s. Euro-American settlers began arriving about a century before that time. These forerunners to many of the local residents that remain today found a different place when they arrived here. A small tribe of Native Americans was living in the Mattole Valley then as they had for about 10,000 years. Their existence here was, surely, not without an effect on the sediment balance. Yet without the mechanical advantage of internal combustion, it is unlikely that their impact matched the scope and intensity of those that replaced them. Their name for the river, which the new settlers adopted, translates to mean “clear water”. Were we to name this river today, would we choose something similar? Could we?

It is out of concern for the implied answer to these questions that the MRC is engaged in reviewing timber harvest plans, reforesting suitable sites, improving poorly constructed roads or decommissioning unneeded roads. We also want to support local residents and landowners who are voluntarily working to restore the sediment balance on their own land. In future issues of the newsletter we will highlight some of those projects so that other landowners will have a better understanding of just what they can do to preserve not only the sediment balance but also this place we call Mattole.



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Table of Contents for Mattole Restoration Newsletter, Issue 18 - Summer/Fall 2002