| 1896 | Trail, Canada facility built as a copper and gold smelter. |
| 1906 | Trail, Canada facility converted to lead and zinc smelter. |
| 1900-1922 | Mining, milling, and smelter activities active in both Canada and the United States, all of which discharge metals into the Upper Columbia and its tributaries. Smelters permanently close at Northport, Grand Forks, Greenwood, and Boundary Falls. |
| 1930s | International Joint Commission holds Trail Smelter in Canada liable for sulfur dioxide air emissions causing property damage in Washington. |
| 1931 | A new Canadian fertilizer plant built to reduce sulphur dioxide emissions becomes a primary source of phosphate discharges into the Columbia. |
| 1942 | Grand Coulee Dam gates close, raising the waters behind the dam 380 feet. Water flows and bank erosion change, affecting depositional patterns of sediment contamination. |
| 1960 | Celgar Pulp Mill begins operation. |
| 1990-1993 | WA Department of Ecology (Ecology) monitors dioxin, furan and trace metal concentrations in suspended particulate matter and fish tissues. The Celgar pulp mill is considered the primary historical source for dioxin and furan contaminants. Celgar plant modernization designed to end discharges of dioxins and furans into the Columbia. |
| 1992 | The USGS conducts a sediment study of Lake Roosevelt describing the transport of metals, the presence of trace metals in bed sediment, and the toxicity of the sediment to benthic invertebrates. |
| 1993 | Ecology initiates studies resulting in Lake Roosevelt being placed on the federal Clean Water Act Section 303(d) list of impaired water bodies. |
| 1994 | Canadian fertilizer plant ceases discharge of phosphate into the Columbia. |
| 1994 | The USGS conducts a fish tissue study to determine the level of mercury and other metals in walleye, smallmouth bass, and rainbow trout. Based on findings, a fact sheet from the Washington State Department of Health (WDOH) advises the public to limit consumption of walleye. |
| 1995 | Discharges of slag from Trail smelter cease. |
| 1997 | Teck implements Trail Modernization Program to further improve quality of effluent (wastewater) being discharged. |
| 1998 | In Canada, wide area human and ecological risk assessment initiated from Castlegar to the U.S. border. |
| 1998 | USGS conducts a follow-up fish tissue study and finds that concentration levels of metals had either not changed or decreased. Ecology identifies temperature, total dissolved gas, mercury, PCBs and pH as parameters for Lake Roosevelt inclusion on the Clean Water Act 303(d) list of impaired water bodies. |
| 1999 | Colville Confederated Tribes petition EPA to conduct an assessment of environmental contamination in Lake Roosevelt under federal Superfund program. |
| 2001-2003 | EPA collects samples of river sediments. EPA recommends a Remedial Investigation Feasibility Study (RI/FS) to assess environmental and human health risks. |
| 2002-2006 | USGS collects data to determine concentrations of trace metals in wind blown dust. |
| 2003 | After negotiations between EPA and Teck Cominco reach an impasse, EPA issues Unilateral Administrative Order under U.S. Superfund law to fund and conduct aspects of the RI/FS. Teck Cominco contests the order and EPA moves forward with the RI/FS using U.S. government funding. |
| 2004 | Canadian government issues a "Diplomatic Note" to the U.S. State Department regarding EPA enforcement order. United States and Canadian governments begin discussions regarding site. |
| 2004 | Colville Confederated Tribes and Washington State ask U.S. District Court to force Teck Cominco to comply with EPA order issued in 2003. |
| 2004 | Ecology updates Clean Water Act 303(d) impairment list for Lake Roosevelt to include temperature, total dissolved gas, and mercury. |
| 2005 | EPA RI/FS studies begin by initiating sediment sampling and fish tissue studies. |
| 2006 | EPA reaches an agreement with Teck Cominco to conduct an RI/FS that Teck Cominco will fund. Teck required to develop EPA approved RI/FS ecological work plan before studies can continue. |
| 2006 | Ninth circuit upholds a district court ruling that Superfund (CERCLA) law does apply to Teck's Trail smelter. |
| 2008 | Lake Roosevelt fish advisory updated. |
| 2009 | RI/FS ecological and human health work plans approved by EPA. Teck Cominco (now called Teck) begins RI/FS studies. |