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Chapter 2 -- Description of the Smith River watershed (continued)

Geology

In the Smith River watershed, elevations range from sea level to 6411 feet (1954 meters) at Bear Mountain. Elevations increase from west to east with the highest elevations along the eastern edge of the watershed, at the crest of the Siskiyou Mountains. Most of the watershed is very steep and dissected. Over 40% of the drainage has slopes greater than 50% (California Department of Fish and Game 1980, Table 12). Glaciated areas are found near the Siskiyou Crest. Gently sloping areas include a few relatively small river terraces around Gasquet and Big Flat and the extensive coastal plain which is about 4 miles (6.4 km) wide. Before flowing into the ocean, the Smith River winds across the coastal plain for about 8 miles (13 km).

Table 12. Area of the Smith River watershed in general slope classes (California Department of Fish and Game 1980).

Percent slope:

Percent of drainage:

0-50%

59.7%

51-70%

28.2%

Over 70%

12.1%

Prior to two million years ago, a flat area existed where the rugged Klamath Mountains now stand. This plain, known as the "Klamath Peneplain", formed during a long period when mountain-building forces were dormant. Roughly one million years ago, mountain-building forces resumed and the Klamath Peneplain was uplifted (Carver, personal communication 1996). During uplift, the surface was simultaneously dissected by erosion, creating a network of steep streams (Tuffly 1995). However, from the highest ridges of the mountains in this area, you can see that the mountain summits all reach to elevations lying in the same plane. This plane, which slopes to the west, forms an upper limit, a ceiling, beyond which the mountains do not protrude (Klaseen and Ellison 1974). The highest peaks and ridges are remnant surfaces of the uplifted Klamath Peneplain (Stone 1993).

Uplift of the Klamath Mountains is related to the movement of tectonic plates within the Cascadia subduction zone (Stone 1993). In the process of subduction, the tectonic plate underlying the sea floor (the Gorda Plate) is colliding with and sliding under another plate that forms North America (Kilbourne and Mualchin 1981). As this has continued for more than 157 million years, various geologic materials on the oceanic plate have been broken off, bent, and added to the edge of the continent. This accumulation includes marine sedimentary and volcanic rocks from the ocean floor which have been metamorphosed to varying degrees.

One of the geologic materials that has been added to the continental margin during subduction is peridotite. This type of rock was created below the ocean floor in the vicinity of diverging tectonic plates. As the tectonic plates under the ocean moved away from each other, rocks were formed from molten materials. The mineral composition of the rocks was dependent on the depth beneath the ocean floor at which they solidified. At deeper levels, peridotite formed which contains high concentrations of magnesium and iron. As subduction has continued over millions of years, peridotite and other materials have been scraped off the ocean plate onto the edge of the continent. Subsequently these accumulated materials have been uplifted, eroded, and broken by faults. During this process, the peridotite has been chemically weathered to varying degrees to produce serpentinized peridotite which is a type of serpentine (McGee-Houghton 1995).

Soils developed from serpentinized peridotite have high concentrations of iron and magnesium and very low amounts of available calcium. These "serpentine soils" also may be deficient in other nutrients in addition to calcium. Also they may have high concentrations of heavy metals and poorly developed soil profiles. Because of these attributes, serpentine soils are unfavorable for most types of vegetation and unique vegetation communities have developed (Franklin and Dyrness 1988). Serpentine soils, also known as ultramafic soils, are found in the Klamath Mountains and dominate the Gasquet Mountain Ultramafic subsection of the watershed. The Klamath Mountains also include marine sedimentary materials and volcanic rock which have been metamorphosed to varying degrees (McGee-Houghton 1995).

Before reaching the Pacific Ocean, the lower Smith River emerges from the mountains onto the Crescent City Plain subsection. This coastal plain is an uplifted marine terrace that is 3.5 to four miles wide. It was formed through the erosive action of ocean waves and was subsequently uplifted. To the east, along the mountains that border the coastal plain, alluvial fans form a surface that slopes to the west. The fans originate from the mouths of several small valleys and merge together to form a discontinuous larger fan (McMillan and Gibson 1987). The short streams and tributaries originating from the first range of hills along the ocean in this area have a tendency to become aggraded (filled with sediments).

The Smith River estuary is located on the northwestern edge of the coastal plain. At the end of the Pleistocene and "during recent times" the Smith River emptied into Lake Earl and then into the ocean (US Corps of Engineers 1971). There are also indications that Lake Earl emptied into the Smith River in recent geologic times (Waldvogel personal communication 1996).

As the Smith River delivers large quantities of sediment to the ocean, wave action carries sand-sized particles to the south. As the wind blows sand off the beach, dunes are created. From Point St. George near Crescent City to the mouth of the Smith River, these dunes form a strip along the coast averaging about a mile wide. The dunes form elongated ridges oriented roughly northwest and some reach 60 feet in height. Many dunes are actively migrating, while others have been stabilized by grasses and other vegetation. Dunes that remain stable eventually become forested with willows, Sitka spruce, and lodgepole pine (US Corps of Engineers 1971). Surface runoff from inland areas has been blocked by the dunes, leading to the formation of Lake Earl and Lake Talawa (McMillan and Gibson 1987).

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