Cowee-Davies

Geology and mining

Cowee-Davies watershed is geologically diverse. In the headwaters, erosion-resistant, monolithic granitic rocks (peach color on map) create spectacular relief. More interesting to miners (and more conducive to fish and forest productivity) are the slates, phyllites and limestones of the rock unit coded Trclt (pale blue). The profitable Eagle River Mine, as well as smaller prospects in Yankee and Cottrell Basins, fell within this rock unit, which also hosted the mining bonanza in Silverbow Basin above Juneau.

Prospectors sought quartz veins striking NW-SE and dipping steeply northeast, as in the above diagram. In the Eocene (early age of mammals), super-heated metamorphic fluids rose through fissures in the bedrock, cooling as they neared the surface and precipitating gold in the veins.

Geology and forests

The largest Sitka spruce grow on valley bottom sand and gravel deposited by streams migrating back and forth across their flood plains for millennia. Extensive flood plains can only develop on “softer” less resistant bedrock that glaciers ground down to low hills. Cowee-Davies has by far the most remaining large-tree flood-plain old growth of any watershed in the City and Borough of Juneau.

Geology and fish

Fisheries biologists consider flood-plain streams the best channel types for fish. Cowee-Davies has far more low-gradient, flood-plain channel than any other stream system on the Juneau roads. Other systems with substantial flood-plain channel–such as Herbert-Windfall and Montana-McGinnis–not coincidentally occupy the same Gastineau Channel fault that underlies Cowee Creek (dashed purple line on geology map). When Juneau was overrun by mile-deep glaciers 20,000 years ago, the heavily fractured rocks in these fault zones were especially vulnerable to erosion. Today, in these long, gentle, fault valleys, superb fish streams meander through flood-plain sediments beneath massive spruce trees. Large fallen logs and productive alder fringe (above right) are a key features of fine fish habitat. But even these biological features ultimately connect back to geology.

Productivity and diversity

Throughout Southeast Alaska and the world, the most productive watersheds are typically developed first. Cowee-Davies watershed is an exception to this rule. In the temperate rain-forest bioregion we could loosely define productive watersheds as those that annually “export” a lot of salmon, and rapidly grow large trees. By those measures, Cowee-Davies excels.

The watershed is not only productive; but exceptionally diverse. Glaciated spires nearly 6000 feet high drain seaward through forest and bog to the most extensive undeveloped coastal meadows in the Juneau area. Much of this habitat diversity and productivity traces back ultimately to bedrock geology, as explained in the left-hand panel.

Aerial comparisons

All of Cowee Meadows was washed by tides at the peak of the Little Ice Age in the 1700s. Land has since risen at a rate of 0.8 inches per year–totalling 5 feet in the 76 years between these photos. The upper shot was taken by the US Navy in 1929. View is to the northeast, from above Akiyama Bight to Cowee Meadows and Echo Cove. Equivalent points on the two photos are numbered for comparison:

1) Extensive young-growth spruce at today’s Echo Ranch was just a few small groves in 1929. 2) Upland old growth in 1929 would be logged by Allen McMurchie, who later donated the Echo Ranch land. Today, this is densely stocked, smooth-canopy second growth. 3) Flats in 1929 remain unforested in 2005; this area is regularly mowed for hay (brighter green). 4) This oxbow widened substantially between 1929 and 2005. 5) Red alder colonized human disturbance alongside McMurchie’s logging road. 6) This stand of open-grown spruces was already tall in 1929, when it was among the northernmost conifers advancing into the meadows. 7) This is one of the most dramatic changes in Cowee Meadows, going from essentially zero forest cover to closed canopy spruce and alder. It’s not part of the higher meadow surface, but an inset river terrace in an abandoned oxbow. In contrast, at . . . 8) there has been little new colonization due to meadow capture, which happens when tall herbs and a thick moss layer leave no exposed soil for conifer germination. 9) Edge of alluvial large-tree forest.

Beaver have been busy in the lower center of the 2005 photo. The round pond rimmed with flood-killed spruces is on the McMurchie Cat Trail. Beaver also dammed and expanded the snakey slough near stand 7, a former salt-marsh channel now above the reach of tides.

Uplift succession and wildlife

The profile below shows an “uplift sequence,” of successively older communities as you move to the right over raised former tideland. Exceptional habitat complexity–forest, parkland, meadow, bog, meandering sloughs, beaver marshes, and lots of ecotones, or habitat edges–translates into very high wildlife values. The meadows host one of Juneau’s last healthy populations of the declining western toad. Deer, black and brown bear, porcupines and marmots graze the rich meadow herbs and sedges. Mink, otter and weasel cruise the stream banks. Mature red alders at the forest edge increase the variety of nesting songbirds. Unusual species such as mountain bluebird, Townsend’s solitaire and trumpeter swan can be seen in migration.