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.

 

Mile 0
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.

 

Mile 0.2
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.

 

Mile 0.4-0.7
There are several small pullouts here with some great views of the confluence of the mighty Copper and Chitina Rivers. The Copper is the only waterway that cuts through the Chugach Mountains, which extend for about 200 miles in a great arc across south-central Alaska. At the confluence, the Copper River is actually smaller than the Chitina River which is considered its tributary. At this point the Copper is about 1/2 mile wide, while the width of the Chitina is over one mile! The Chitina carries more water year-round than the Copper. Even though the Chitina River drains a smaller area than the Copper, it has more runoff due to greater precipitation in its watershed.

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.

 

The Copper River, Fishwheels, and Mount Drum.

 

Mile 1.2 Copper River Bridge
High peaks of the Wrangell Mountains are visible to the north. Suspended sediment loads of the Copper and Chitina Rivers are high, but the dissolved sediment loads are low; these rivers are dirty...but not polluted. Millions of tons of natural sediment, mostly silt and clay are carried downstream each year.

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.

 

Mile 1.7
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).

 

Look for the brightly colored deposits of a large volcanic debris in the bluffs along the Kotsina River.

 

Mile 2.0-2.5
There are several small turnouts from which the lower Kotsina River can be seen. The Kotsina, a moderate size tributary of the Copper River, gets much of its water from Long and Kluvesna Glaciers which drain southward off Mt.Wrangell.

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.

 

Mile 4.9
Rocks poorly exposed on the north (left) side of the road are basalt and greenstone of the Nicolai Greenstone rock unit. These rocks originated as flows of basaltic lava during the Triassic Period, about 220 million years ago. Later they were partially altered by heat and pressure to form the greenstone that is widely distributed in the Wrangell Mountains and is the source of copper mined near Kennecott.

Mile 5.0-5.4
Limestone is exposed on the north side of the road and there are several small pullouts that offer nice views of the Chitina River and Chugach Mountains to the south. We have obviously left the Kotsina River drainage and entered the lower part of the Chitina River Basin.

 

Fed by glaciers, the Chitina is a classic example of a braided river

 

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.

Mile 15.5-16.0
The road runs southeast here in a low valley between two long ridges of gravel. These gravel ridges are good examples of lateral moraines that were deposited along the margin of a large glacier as it receded.

Mile 17.1-17.3
A few turnouts on the south side of the road offer good views of the Kuskulana River and bridge. Exposures on the other side of the road show the type of sediments that make up glacial moraines.

 

Mile 17.4
Kuskulana River Canyon and Bridge. This is the only structure on the McCarthy Road that was originally built in 1910 as a railroad bridge and is still in service as as automobile bridge. It was the only railroad bridge in this area constructed of steel girders that span the canyon rather than timber pilings driven into the streambed. Note that the bridge is supported primarily by the metamorphic bedrock of the inner gorge rather than the thick layer of glacial gravels near the surface. The Kuskulana’s muddy waters reflect its origin from the melting of several glaciers that drain off the southern and western slopes of Mt. Blackburn.

 

The Kuskulana Bridge is perched 238 feet above the raging Kuskulana River.

 

Mile 20-21
Vegetation and poor drainage in this area are influenced by the presence of permafrost at relatively shallow depths. This is a good place to contemplate the problems of road construction and maintenance in areas of permafrost, muskeg and swamps.

Mile 24
Gilahina Butte to the south (right) of the road is composed of gabbro and gneiss; its sides have been smoothed by large glaciers that formally flowed down the Chitina Valley.

Mile 26.8
Clear waters of the Chokosna River support salmon spawning beds.

 

A classic reminder of a bygone era, this impressive trestle was constructed in eight days during the winter of 1911.

 

Mile 29.3
Gilahina River. Remnants of the C.R. and N.W. trestle cross the valley high above the current road. It is interesting to compare this trestle to the Kuskulana Bridge built about the same time.

Mile 34.7
Crystalline Hills trailhead. Follow this 2.5 mile loop trail to the base of the igneous Crystalline Hills. There are some nice views of the Chitina River Valley, Moose Lake, and Nelson Mountain and its landslide in the distant Chugach Range to the south.

 

 

Mile 35.2
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.

 

Mile 44.5
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.

 

Sockeye salmon spawn in Long Lake from September through April!

Mile 45.2
Many salmon migrate from the ocean through the muddy Copper, Chitina, and Lakina Rivers to spawn in the clear waters of this small stream and Long Lake. Glacial till and gravels deposited by ancient glaciers and glacial streams mantle the slopes around the lake.

Mile 48.3
Looking across the Long Lake inlet streams the remnants of a C.R. & N.W. Railway trestle can be seen. Marble and limestone is exposed at the trestle footings and in road cuts nearest the stream. The road cuts through sandstone and conglomerate as it ascends the east side of the stream valley.

Mile 56.7
Swift Creek, a small clearwater stream drains off Fireweed Mountain. Steeply dipping mudstones of the Chititu Formation are exposed in the cutbank on the west side of the stream. Fireweed Mountain is made up of these dark-colored mudstones and light-colored, silica-rich igneous rocks.

 

Kennecott River valley

Mile 58.1-58.4
The Kennicott Glacier and River can be viewed from several small turnouts on the southeast (right) side of the road. Ice of the glacier is mantled by rock material, ranging from large boulders to very fine glacial flour that is still in transit. A short walk around the glacier terminus will make it obvious why glacial streams are so muddy. The Kennicott River runs from the glacier down to the Nizina River, so it is only five miles long. Bluffs along the Kennicott River apparently reflect down-cutting by stream erosion after the glacier melted back from its earlier, more extensive size. It can be seen from these viewpoints that the river valley gets progressively deeper going downstream. The Kennicott River drops over 250 feet in its five mile run between the glacier and the Nizina River. Where it empties into the Nizina, the Kennicott River is in an erosional canyon over 350 feet deep; ten miles further downstream the Nizina is entrenched over 600 feet where it joins the Chitina River.

Mile 58.5
National Park Service McCarthy Road Information Station. Open daily during summer months. Park Rangers and park volunteers are available to help plan your visit to the McCarthy/Kennecott area.

Mile 59.2 – Road's End.
Access to McCarthy is by a footbridge across the Kennicott River. The bridge is open to foot and bicycle traffic only. Once across you can continue to the town of McCarthy (½ mile), or the Kennecott Mines National Historic Landmark. See "how to get to Kennecott/McCarthy" or the McCarthy Road Guide for more information. This area is overflowing with history and adventure!