Repairing Slate Creek: What Mining Left in Denali's Kantishna Hills

A stream can look repaired and still be contaminated. At Slate Creek in Denali National Park and Preserve, two clean-ups cut erosion until the water met turbidity standards, yet antimony, arsenic, cadmium, copper and iron downstream stayed many times above levels known to harm aquatic organisms, and a flood in 2014 undid the channel built to keep the creek off the old mine. Paul Burger, Chad Hults and Shannon Coykendall of the National Park Service and Andrew Kirby of Arizona State University describe what the park learned and how it now plans a more lasting repair.

The Slate Mine site looking downstream, with the active channel running through the mine tailings; the row of wooden spikes behind the helicopter marks the edge of the abandoned constructed channel (NPS photo).

How a stampede became a stream problem

In 1903 James Wickersham and several others tried to make the first ascent of Mount McKinley and failed, but on the way back to Fairbanks Wickersham found gold in stream gravels in the northern Kantishna Hills, in what is now Denali. The following year gold turned up on Friday and Eureka creeks and several thousand miners stampeded in. Most left by early 1906, since only a few streams carried paying quantities of gold. Meanwhile galena, stibnite and silver drew some prospectors to lode deposits, and in 1905 a rich antimony deposit was found in upper Slate Creek. More than 325 tons of ore were mined between 1905 and the end of World War II. Antimony is a strategic metal used for hardening lead and for semiconductors, diodes, batteries and cable sheathing.

The deregulation of the price of gold in 1972 brought a resurgence of placer mining in the 1970s and early 1980s, with heavy equipment on Moose Creek and mostly hand tools on Eldorado Creek, one of its tributaries. When Denali was expanded in 1980 the mines and placer deposits fell inside the park and major mining ended, though placer mining continues. The authors add that people involved in mining, geology and transportation helped bring the area's scenery and wildlife to public attention, and that widely publicized mountaineering expeditions and the naturalist Charles Sheldon's early efforts led Congress to establish Mount McKinley National Park in 1917.

The Kantishna area, showing major streams and the Slate Creek Mine (map from the Park Service's article).

What mining did to the water

Mining channelized Eldorado Creek, which reduces habitat for aquatic insects and for fish spawning and rearing, and removing large rocks and natural debris made the habitat less complex, which can reduce aquatic biodiversity. Land stripped of vegetation erodes easily, and the fine sediment that washes into streams can fill the gravel pores that developing fish eggs and insects rely on. Scientists measure turbidity, how easily light passes through water, to describe the suspended fine sediment; higher turbidity generally means fewer invertebrates, the base of stream food webs, and can also kill or harm fish and drive them away.

In 1994 the Environmental Protection Agency designated Slate Creek an impaired waterway, from its headwaters to its meeting with Eldorado Creek, because erosion of old mine tailings put too much sediment in it. Reclamation in 1997-1998 and again in 2010 removed mining debris and tailings from the floodplain, re-contoured slopes and rebuilt the channel. Erosion dropped dramatically, and testing in 2008-2011 showed the stream meeting water quality standards for turbidity. The same testing found the metals above.

Slate Creek before reclamation (June 1, 2010, left; USGS) and after (September 2010, right; NPS).

What happened to the grayling

Electrofishing surveys in 1982 were the first in the Slate and Eldorado creek basin. Eldorado Creek below its meeting with Slate Creek held few Arctic grayling, averaging 4.6 fish per kilometer, but the creek above the meeting had the highest grayling density of any stream sampled in the Kantishna Hills, 33.3 fish per kilometer, which suggested the fish could still pass through the poor habitat below to reach it. The following year the Slate Creek Mine became active again, and a survey found fewer grayling in lower Eldorado, especially from 300 feet downstream of the confluence. There were no more surveys until 2017.

Park surveys of Eldorado Creek began again in 2017, while suction dredging was active in the creek, and found no fish upstream of Slate Creek. That stretch still has excellent physical habitat, good water quality and a thriving community of aquatic insects, and there were no long-term physical barriers to migration, with streamflow and water temperatures similar to those of the 1980s. Below the confluence the channel is badly disturbed, and Slate Creek and naturally mineralized springs along Eldorado Creek keep adding heavy metals such as arsenic, antimony and iron. The authors say destruction of habitat and poor water quality are the main factors in grayling presence, abundance and movement, and both may be improved by restoration.

A flood and a new approach

In spring 2014 a massive rainstorm washed out roads and changed the course of many streams in the Kantishna area. At the Slate Creek mine site, the channel built to keep water away from the main ore deposit failed completely, three years after it began to show signs of deterioration. The stream went back to where it ran before reclamation, and many erosion-control structures washed away.

Stream restoration has evolved quickly, the authors say. Planners now look not only at a stream's shape and pattern but at its function, meaning the physical, chemical and biological processes at work. In 2017 the park, working with the Bureau of Land Management and Stream Mechanics, PLLC Inc., surveyed unmined streams in the watershed, measuring width, depth, area, bed material size and slope to find how much water each could carry, and judging channel stability and habitat from features such as the spacing of pools and riffles. The same measurements were repeated on mined or modified stretches of Slate Creek, so the difference shows how much has changed and how a restored channel should look. Several alternative designs are being evaluated for cost and feasibility, with the hope of restoring natural function and helping native aquatic species recover.

The authors see the project as a model for evaluating the park's other major mined sites. Knowing how well each stream can recover will help the Park Service put its effort where intervention is needed and leave alone streams that are recovering naturally, where disturbance might do more harm than good.

Sources

• Paul Burger, Chad Hults and Shannon Coykendall (National Park Service) and Andrew Kirby (Arizona State University), Eldorado: Treasure, tragedy, and triumph in the Kantishna Hills, Alaska Park Science 19(1), National Park Service, 2020; rewritten in hubnx's own words.