For 100 years, a dam trapped 3 million cubic yards of toxic mining waste in Montana. Engineers removed it; what happened to the river next was remarkable

For nearly a century, a dam on Montana's Clark Fork River held back millions of cubic yards of contaminated mining waste. When engineers started removing the structure back in 2008, they were not just dismantling an ageing dam but they were also adressing a long-standing threat to the river and the communities downstream.

A decade after the Milltown Dam's removal, the river had developed a different pattern of channels through the former reservoir area. According to Missoula County officials, this new arrangement of winding and branching channels offered better flood control than the old dam.

How mining waste accumulated behind the dam

The Milltown Dam stood at the confluence of the Blackfoot and Clark Fork rivers, east of Missoula, Montana and was built to generate hydropower, it was around 100 years old when its removal began in 2008.


Over decades, mining operations upstream left contaminated sediments that accumulated behind the dam. By the time removal work started, the reservoir held approximately 3 million cubic yards of mining waste.

The contaminated material was a risk because floods and other high-flow events could disturb the sediments and carry them downstream. The ageing dam itself also was a potential danger if it failed during extreme water levels.

The cleanup work and removal project followed years of debate over the dam's future and how to address the toxic material trapped behind it.

Why keeping the dam would not have prevented flooding

The dam's removal came under scrutiny in May 2018 once again, when Montana experienced exceptionally high water levels on the Clark Fork River. Some residents in Missoula's Orchard Homes neighbourhood wondered whether the old structure might have reduced flooding.

However, officials and river experts said this overlooked the dam's original purpose.

According to Montana Public Radio's May 2018 report, John DeArment, science director at the Clark Fork Coalition, explained that the structure was a run-of-the-river dam rather than a reservoir designed to store floodwaters.

Missoula County environmental health specialist Travis Ross made a similar point in comments reported by KTVH. The dam had not been built to provide measurable flood protection, and the contaminated sediments had substantially reduced the reservoir's remaining capacity.

The structure had also faced serious problems during earlier floods. During the 1908 flood, which produced a water level of around 17 feet, a substantial part of the dam's spillway was reportedly destroyed.

Officials were concerned that another major flood could cause the structure to fail, releasing contaminated sediments into the river and carrying them through Missoula and downstream communities.

What changed after the dam was removed?

The removal allowed the river to establish a different course through the former Milltown Pond area.

As described by Ross in KTVH's report, the river developed meanders and braided channels, creating a more varied channel pattern through the site. County officials said this arrangement was better for flood control than relying on the old dam.

The change was big because it addressed two connected problems: the threat posed by contaminated sediments and the limitations of an ageing structure that had never been designed for flood control.

The river's new channel pattern also showed that removing a dam can change how a river moves through a landscape, rather than simply eliminating a barrier.