Juneau’s working watershed
Gold Creek drains one of the most intensively altered watersheds in Southeast Alaska. Gold, silver and lead were mined from this basin for 60 years, and the well field in Last Chance aquifer still provides most of the drinking water for downtown Juneau and Douglas Island. Added to these human impacts is natural dynamism; steep, unstable slopes are so active that conifers occupy only 12% of the watershed, compared to 80% in a more stable basin such as Fish Creek.
Although Juneau’s weather can be trying at times, many of us choose to live here for the quick access on foot to scenic trails and spectacular high country. Where else can you watch mountain goats from the sidewalk, or lie in a field of alpine wildflowers only minutes from city center? To better appreciate the uniqueness of Gold Creek watershed, here’s some background on geology, habitats and wildlife.
Bedrock geology, topography, hardrock mining
Gold Creek watershed is transected by a series of bedrock belts–color-coded above–running roughly NW-SE. The dominant bedrock faults–heavily fractured shear zones–are also aligned NW-SE; for example, the Sumdum thrust (dashed orange line). This major fault splits the entire City and Borough of Juneau, gradually converging with the Gastineau Channel fault separating the mainland from Douglas Island. A third fault–the Silverbow–slashes east-west obliquely across the Gastineau and Sumdum. During the great ice ages, mile-deep glaciers excavated the weaker, more shattered rocks in these shear zones, creating long fiords such as Gastineau Channel, and shorter “pocket valleys” such as Gold Creek watershed.
Most of the gold mining was concentrated within the rock unit coded pale blue, composed of slate, schist and limestone. Gold-bearing quartz veins thread this slatey matrix, symbolized by white lines in the profile view through line A-B at upper right. The bedrock matrix is Triassic–early age of dinosaurs, about 200 million years old. But the gold itself was not emplaced in these or adjacent rock units until much more recently.
In Eocene times (early age of mammals, about 55 million years ago) the Juneau area experienced severe geologic stresses from changing plate movement. Over a span of only 1 to 2 million years–an eye-blink as geologists reckon time–hot water forced from rocks deep in the earth percolated upward through the permeable fault zones. Cooling in the upper crust, this fluid precipitated gold and other minerals in the resulting veins. It’s no coincidence that the Alaska-Juneau and Perseverance mines were situated near the intersection of the Silverbow fault (E-W) and Sumdum thrust (NW-SE).
Landslides, alluvium, placer mining
Stream deposits, or “alluvium,” are stippled on the bedrock map at left. Even before miners began tunneling into bedrock, placer operations targeted these deposits at Last Chance and Silverbow basins. The Last Chance placer gravels are keyed to an ancient domed slide deposit (profile below) where today’s Basin Road crosses Gold Creek. Beneath these alluvial gravels, as explained in Spencer (1906), “the heavy gold tended to cling to the bottom and lodge in the crevices in the bedrock. In the area covered by the [former] pond, bedrock concentration would not take place.”
Yet another landslide off Mount Juneau briefly dammed Gold Creek, forcing it to loop southward through soft ancient delta deposits in what became Evergreen Bowl. When the creek later mined through the slide rubble and straightened out again, an abandoned oxbow pond remained (since filled in). The bowl’s 100-foot walls testify to the power of stream erosion.
The Mount Roberts trail accesses lush subalpine meadow and parkland communities near the tram landing, and higher alpine tundra vegetation on Gold Ridge and beyond. Interpretive signs on this trail explain local subalpine and alpine ecology.
Unstable hillsides, fickle bottomlands
To a Southeast naturalist, the most striking feature of Gold Creek watershed is the paucity of conifers. A steady barrage of snow and rock avalanches–particularly common in the mining decades when logging reduced slope stability–discourages brittle-trunked spruce and hemlock. Even in the absence of slides, slow downhill creep of soggy snowpack and loose, saturated soil inhibits conifers.
Hemlock-spruce forest stands out on the center-panel air photo; it’s darker green and coarser textured than the shrubby, steep-slope communities dominated by Sitka alder (Alnus crispa). The lower Mt Roberts Trail transects the only extensive conifer forest in the watershed.
Conifers are also sparse in the bottomlands. Most of the valley floor was reworked by hydraulic mining in the late 1800s. But even before that, Gold Creek and its tributaries were dynamic and shifty, constantly responding to rock and mudslides as described on the left-hand geology panel. Conifer colonization is patchy and sporadic on highly active alluvium.
Plants with a preference for lime-rich substrates are called “calcicoles.” In Southeast Alaska there are few obligate calcicoles, but some plants do especially well on limey rock. On Perseverance Trail, watch for maidenhair and holly ferns (Adiantum pedatum, Polystichum braunii), purple mountain saxifrage (Saxifraga oppositifolia) and columbine (Aquilegia formosa). The band of slate and limestone (pale blue on the geology map) is one of the better places to look, but limey rocks are not restricted to this unit.
Oreamnos americanus should probably not be called the mountain “goat.” As this family tree shows, it’s more closely related to the European chamois and even the bighorn sheep than to the familiar domestic goat. Oreamnos is the only large mammal who lives year-round in the extreme conditions of Alaska’s coastal mountains. In Juneau, the easiest time to see these ultimate alpinists is late winter and early spring, when they descend to low elevations on the brushy slopes above town. With binoculars, look for nanny-kid pairs. Young goats are utterly dependent on their mothers for survival in the deep winter snows. When new kids are born in May and June, the yearlings are dismissed. But by this time the snows have melted and southerly mountain slopes are full of choice green forage.
A century of change in the Gold Creek highlands
View northeast from Gold Ridge to the watershed divide. Both photos were taken at summer’s end, when only glaciers and permanent snowfields remained. In the south-facing bowl below Olds Mountain there has been notably little change in permanent snow cover. 1) Bare ice (grey) in the 1902 photo may have been the last vestige of a small glacier, formed during the Little Ice Age. 2) A patch of Sitka alder–not present in 1902–has developed on convex terrain. 3) Twin Summit Ridge, just outside the Gold Creek Watershed, was capped by much larger snowfields in 1902. 4) By 2009, there was no trace of the 1902 glacier that filled the deep gully of the Silverbow fault, shaded by 4050-foot Clark Peak, just to the right of this photo.
Trail AccessMt. Roberts Trail
Distance: 3.7 miles.
Highest elevation: 3200 ft
Ownership: CBJ FS private
Notes: Forest limit–~1800 feet–also accessed by aerial tram. Most intensive use of any Southeast high country.
Mt. Juneau Trail
Distance: 1.7 miles.
Highest: elevation 3576 ft
Ownership: CBJ, private
Notes: Seasonal trail only; traverses avalanche paths. Major goat habitat.
Distance: 3.3 miles.
Highest: elevation 1100 ft
Ownership: CBJ, private
Notes: Easiest gradient of these 4 trails. Perseverance mine operated 1886 to 1943.
Granite Creek Trail
Distance: 2.3 miles.
Highest: elevation 2300 ft
Notes: High subalpine bowl. Granitic bedrock is unusual on Juneau trails.
The Mt Roberts, Mt Juneau & Granite Creek trails, under multiple ownership, are managed by the AK Department of Natural Resources.
• Please do not pick or dig up
• Do not feed wildlife.
Pack it out
• Carry out all trash.
• Please pick up after your pet.