Streamside Tailings Operable Unit
The cleanup of the 22-mile stretch of Silver Bow Creek between Butte and Warm Springs is a unique case in Superfund history. Unlike most sites where the Responsible Party does the physical work, the State of Montana acted as the general contractor using ARCO's money.
What are streamside tailings?
"Slickens" are deposits of mine tailings that were washed downstream from Butte by the massive 1908 flood. These tailings spread across the Silver Bow Creek floodplain, creating barren, orange-to-white stained areas where nothing could grow.
The tailings were toxic to vegetation and prevented fish and wildlife from living in the creek due to high concentrations of copper, cadmium, lead, and zinc.
State characterization (Early 1990s)
In the early 1990s, the State of Montana (then the Department of Health and Environmental Sciences, now DEQ) took the lead in studying the creek. They performed the initial "characterization" of the contamination.
The state's research identified millions of cubic yards of slickens and documented how the tailings were preventing anything from living in or near the creek.
The 1995 Record of Decision
The EPA eventually asserted federal authority, officially naming ARCO as the Primary Responsible Party (PRP). The EPA oversaw the formal Remedial Investigation/Feasibility Study (RI/FS), which relied heavily on the state's earlier data but formalized it under federal Superfund law.
In 1995, EPA and DEQ issued the Record of Decision (ROD). It bypassed "capping" (leaving waste in place) and instead mandated a "Remove, Restore, and Replace" strategy: dig up the tailings, move them to a repository, and rebuild the creek channel.
The 1999 Agreement: DEQ takes the lead
ARCO initially fought the total removal plan because of its high cost. However, a landmark settlement was reached in 1999 — the Streamside Tailings Consent Decree.
The handover: The parties agreed that the State of Montana (DEQ) would actually perform the cleanup work instead of ARCO.
The funding: ARCO paid the state $80 million (plus interest) specifically for the SSTOU remediation. This money was placed in a dedicated account for DEQ to draw from to pay contractors and manage the project.
This arrangement — where the state performs the work using responsible party funds — is unusual in Superfund history.
Contingency funding
Contingency funding was a critical part of the 1999 and 2008 settlements to ensure the work didn't stop if the initial $80 million ran out — which it eventually did, since the volume of waste was higher than estimated.
Initial funding: $80 million from ARCO set aside for DEQ's use.
The agreement: If DEQ performed the work efficiently but costs exceeded the initial amount, additional funds from other ARCO settlements could be shifted to cover overruns.
Round robin provision: In later phases (like the 2008 Clark Fork River settlement), contingency funds were established. If one cleanup area was completed under budget, those funds could rotate to cover overruns in other areas. Conversely, if costs increased significantly, ARCO/BP was required to provide additional funding to ensure the Record of Decision was fully met.
How the cleanup was done
The cleanup was divided into four sub-areas. DEQ started at the headwaters (Sub-area 1 near Butte) in 1999 and worked downstream toward Warm Springs.
Who did the work: Local Montana contractors hired by DEQ.
Where the waste went: Much of the contaminated material was hauled by train to the Opportunity Ponds — the ARCO Waste Management Area.
The result: Over 3.3 million cubic yards of tailings have been removed, and the creek now supports fish and wildlife for the first time in over a century.
What Silver Bow Creek looks like today
Where contaminated slickens once prevented any life, Silver Bow Creek now flows through a reconstructed channel with vegetation along its banks. Fish have returned to stretches of the creek that had been lifeless for generations.
While monitoring continues, the transformation from an industrial conduit to a functioning stream is visible along many reaches.
Long-term monitoring
Cleanup does not end with construction. Ongoing monitoring tracks water quality, assesses channel stability, ensures vegetation establishment, and detects changes after flood events.
Work continues downstream on the Clark Fork River Operable Unit (CFROU), addressing contamination spread by the 1908 flood across the broader floodplain.