What 80 years of mining did to Lake Coeur d’Alene

Second in a series.

Because of ongoing disputes and concerns, Lake Coeur d’Alene is the subject of a study by the National Academy of Sciences. This is the second in a series of articles about the lake’s environmental health and how that affects business and land use in the region.

The ancestors of the Schitsu’umsh once lived along the shores of Lake Coeur d’Alene. Father Nicholas Point, who ran the Coeur d’Alene Mission, described the tribal members filling their canoes with fish after just a handful of hours of fishing. A hundred hunters would return with 600 deer. Difficult to reach, the Coeur d’Alene basin was an isolated and mountainous idyll, inhabited by a few thousand.

True to the meme of the West, settlers invaded and displaced the Schitsu’umsh when in 1883, A.J. Prichard discovered gold in a tributary of the North Fork of the Coeur d’Alene River. The gold rush brought 5,000 prospectors up the river, looking to make a quick buck.

Gold mining on the North Fork lasted 50 years but it wasn’t what put the Coeur d’Alene on the mining map. The discovery of silver-lead-zinc deposits along the South Fork of the Coeur d’Alene River and its tributaries in 1884 marked the beginning of base metal mining in what would become known as Silver Valley. The Coeur d’Alene district produced more than 1.1 billion ounces of silver, more than any other mining district in the United States. It remains one of the three largest producers of silver in the world.

Mining operations stretched along the Coeur d’Alene River and its tributaries. Tailings from processing ore were dumped directly into the river and the creeks feeding it. The sediment load and frequent floods permanently changed the river and creeks, creating new channels and moving bars of tailings, especially along the South Fork. Instead of the original soil and rocks, the banks of the river and creeks became lined with banks of deposited tailings. Sediment loading into local surface waters was exacerbated by timber harvesting, which stripped the hillsides around mines of their vegetation. The timber was used for framing mine shafts and adits, and for fuel until replaced by coal and oil.

By 1900, the tailings reached the lake and affected farms and livestock downstream. Because of the complaints, mine operators began the installation in 1901 of plank dams on the South Fork to catch the tailings. This did not stop farmers downstream from beginning what became many decades of lawsuits to stop the disposal of tailings into the river and its tributaries.

The plank dams were not maintained. A flood in 1917 destroyed two of them, sending a huge load of liberated sediments downstream towards the lake. Another failed from a flood in 1933, with the same effect. Once breached, the dams were not replaced because their impoundments were already full of sediment — they would not be effective in capturing further tailings even if they had room. Nor had they prevented farmers from filing more lawsuits — an activity that lasted decades.

According to a previous study by the NAS: “The farmers’ problems undoubtedly were exacerbated by the damming of the Spokane River at Post Falls in 1906, which raised the level of Lake Coeur d’Alene, flooding the lower reaches of the Coeur d’Alene River and, as a result, increasing the rate of deposition and causing the river to flood over its banks and deposit tailings on the surrounding lands more frequently.”

By the mid-20th century, advances in mineral processing used ore crushed into very small particles. Smaller particles travel much further, so this made it easier for mine wastes to travel farther downstream and into the lake. The only upside to this was that new extraction methods greatly reduced the amount of metals left over in these wastes. So sediments traveled further but were less contaminated with undesirable metals like zinc, cadmium and lead.

Not only did the old style of devil-may-care mining contaminate streams, it made it easier to move mine wastes further into the lake. Metal-rich emissions from the Bunker Hill smelter and fuming-zinc extraction plants also spread metals contamination onto soils and into surface waters through the air. All of these effects added more metals-contaminated material onto the bed of Lake Coeur d’Alene.

Both the Coeur d’Alene River bed and the lake bottom contain millions of tons of metal-bearing mine wastes. The 2005 NAS study reported the lake bottom is host to over 83 million tons (75 million metric tonnes) of contaminated sediments washed down during the 84 years of disposal into the South Fork and its tributaries. In 1968, disposal of mine wastes into the river was halted by the advent of environmental regulations. Water quality and soil contamination have improved since then. A number of factors contribute to this: the cessation of mine waste disposal into surface waters, the Superfund cleanup of mine sites along the Silver Valley and the stabilization of tailing-rich river and stream banks upstream of the lake.

Lake Coeur d’Alene is very deep. For now, most of the contaminated material sits quietly on the lake bottom. The current environment prevents the liberation of the undesirable metals in the mine-derived sediments – or so most of the environmental scientists who study the lake believe. According to the 2009 Coeur d’Alene Lake Management Study: “Metals present today in the lake sediments, water, and certain flora and fauna, include: mercury, copper, silver, cadmium, arsenic, lead, zinc, antimony, iron, and manganese.” If the sediments remain undisturbed, the metals they contain present little problem to aquatic life and human use of the lake.

That’s a big “if,” unfortunately. The impact of increasing human use of the lake may be increasing the phosphate and nitrate concentrations in the lake waters. These nutrients can lead to eutrophication, undesirable algal blooms and a change in the amount of oxygen at the bottom of the lake. The possibility of changing the oxygen content is not good, because that change can mobilize the metals out of the lake bottom sediments. Once that happens, the effect on the lake and its wildlife will be much worse than the current fish advisories.

To put this all into a sound bite: Millions of tons of mine wastes from North America’s most productive silver district could poison Lake Coeur d’Alene if human use of the lake continues to add nitrate and phosphate nutrients into the lake waters. Why does this matter? Because the health of the tourism-driven economy in the Coeur d’Alene region depends on the health of the lake.

Next week: Land-use effects on Lake Coeur d’Alene.

This article includes a correction: the previous version stated that higher-oxygen levels could release metals from lake-bottom mine wastes, which is incorrect. In reality the oxygen cycle in Lake Coeur d’Alene is complex and the release of metals would involve lower levels of oxygen at the lake bottom. The article also replaced a shorter list of elements of concern in the lake sediment with a more detail list from the 2009 Coeur d’Alene Lake Management Plan. IBR thanks the IDEQ for their help in clarifying these items.