McCarthy Road Geology
The McCarthy Road follows an old railroad bed from Chitina for about 60 miles east almost to McCarthy. The Copper River and Northwestern, or C.R. and N.W., Railway was built between 1909 and 1912 to serve the Kennicott copper mining area. Despite early claims that C.R. & N.W. stood for "Can't Run and Never Will," the railroad operated successfully until it was abandoned when large scale mining ended in 1938. Ice flows and spring floods destroyed the Copper River Bridge only a few years after maintenance of the rail line ceased. Most of the rails were salvaged for scrap iron. The route was little used from 1938 until 1971 when it was modified into an automobile route by construction of a new bridge over the Copper River and road surface work along the way.
The McCarthy Road begins where the Edgerton Highway intersects the old railroad bed near the lake in downtown Chitina. A good way to start your trip is with a visit to the nearby Chitina Ranger Station. Current information about the route, road conditions, the park, hikes, fishing opportunities, and other essential topics can be obtained there.
Just beyond Town Lake, the road goes through a deep narrow gap known locally as the "railroad cut." Actually it was originally a tunnel and was later altered into an open roadcut. The rocks exposed here are schist and phyllite typical of the Chugach Range.
The Copper and its tributaries, including the Chitina, drain an area that covers approximately 24,000 square miles. Much of this drainage basin lies within Wrangell-St. Elias National Park and Preserve and about 3,500 square miles, or 17% of the drainage basin, is covered by glaciers. Because of this glacier influence, high water in the Copper River typically occurs not during the snow melt of spring, but during summer hot spells that cause rapid melting of ice. Low water usually occurs in late winter when everything is frozen.
In 1950 the U.S. Geological Survey established a stream gauging station at the head of Woods Canyon, about 3.5 miles downstream, well below this confluence. Records from that gauge show that the average discharge of the Copper River has been 37,510 cubic feet per second (CFS) or slightly over 27 million acre feet per year. The largest recorded flood since 1950 was on August 8, 1981, when the river discharged over 380,000 CFS at the gauge. The lowest flow was recorded in March of 1956 when the river carried only 2,000 CFS at the gauge.
Rocks exposed on the west (left) side of the road between here and the bridge are gneiss, schist, and phyllite with some quartz veins.
Mile 1.2 Copper River Bridge
The muddy waters that result from all this suspended sediment create an aquatic environment that is not very favorable as a permanent home for most fish. Therefore the Copper and Chitina Rivers have rather small resident fish populations. They do, however, have fantastic populations of migrating fish during the times that mature salmon "run" up to clear-water spawning beds and juveniles run down to the ocean each year. The muddy waters of the Copper allow successful salmon fishing by the use of dip nets and fishwheels; neither of these methods would work in clearwater streams.
Sign indicating that the McCarthy Road enters Wrangell-St. Elias National Park and Preserve at this point. Turnoffs lead to a campground on the south (right) side of the road and to the Copper River and Kotsina River delta on the north (left).
Brightly colored deposits of a large volcanic debris flow are exposed on the south side of the road and can be also seen in the Kotsina River bluffs a mile to the north. Field studies show that these deposits came from great volumes of ash and clay that were originally high on Mt. Wrangell. They became liquefied when volcanic activity produced much steam and also caused melting of snow and ice. Movement probably began as a volcanic mudflow, but as the mudflow raced down steep slopes of the mountain and along the Chetaslina River it ripped out large pieces of bedrock and picked up loose blocks of rock and river cobbles to become a volcanic debris flow. Materials from this event are called the Chetaslina Volcanic Debris Flow and they were deposited along the Copper Valley and in the lower portions of its tributaries from the Tonsina to the Chitina. These debris flow deposits are overlain in various places by lava flows and also by stream, lake, wind, and glacial sediments. Furthermore, they have experienced considerable erosion by stream activity and by a major glacial advance that occurred after the debris flow. Radioactive potassium-argon dating of a lava flow which overlies debris flow deposits indicates that the Chetaslina Volcanic Debris Flow took place about 200,000 years ago.
Streams throughout the world exhibit only three basic patterns in their channel form: straight channels, which are uncommon in large streams; meandering channels, which consist of many curves and loop-like bends; and braided channels. The Chitina is a classic example of a braided river as are many of the other streams in this region. Braided streams are characterized by many dividing and re-uniting channels and by numerous islands and gravel bars. The braided channel pattern tends to develop in streams that; a (carry a lot of sand and gravel, b) have fairly steep slopes, or gradients, and c) undergo frequent fluctuations in water level. The Chitina River meets these conditions; in the area we can see here it drops about 13 feet per mile, which is steep for such a large stream. Glaciers provide the stream with a great deal of sediment ranging from fine clay to boulders. Weather patterns cause flow variations because the river rises during warm and/or wet weather and drops during cold and/ or dry weather. Consequently, much of the sediment being transported to the sea is temporarily stored as islands or bars of gravel, sand, and mud along the Chitina River. If you happen to be viewing the river after several days of hot weather, most of the islands and bars will be flooded.
The turnout to Moose Lake is on the south side of the road. The Crystalline Hills north of the road are composed mostly of gneiss and gabbro, but the lower slopes just north of Moose Lake are made up of a light colored marble.
The Lakina River gets some of its water from melting glaciers. The bridge was originally built across a river on the Glenn Highway but later removed during a highway upgrade project. Still later it was re-assembled on the Lakina. The bridge itself has functioned well in this location, but it is obvious that a much longer structure would be preferable. There have been repeated erosion problems and washouts of the long eastern approach to the bridge.
Mile 59.2 – Road's End.