Tenmile Creek (Lewis and Clark County, Montana)
| Tenmile Creek | |
| Country | United States |
|---|---|
| State | Montana |
| County | Lewis and Clark County |
| Source | |
| - location | East side of the Continental Divide in southern Lewis and Clark County, Montana |
| - elevation | 7,120 ft (2,170 m) |
| - coordinates | 46°25′36.7392″N 112°18′50.3706″W / 46.426872000°N 112.313991833°W |
| Mouth | Prickly Pear Creek |
| - location | Lewis and Clark County, Montana |
| - elevation | 3,663 ft (1,116 m) |
| - coordinates | 46°40′24.5382″N 111°58′32.973″W / 46.673482833°N 111.97582583°WCoordinates: 46°40′24.5382″N 111°58′32.973″W / 46.673482833°N 111.97582583°W |
| Length | 26.5 mi (43 km) |
| Basin | 200 sq mi (518 km2) |
  Location of the headwaters of Tenmile Creek in Lewis and Clark County, Montana
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Tenmile Creek is a 26.5-mile (42.6 km) long tributary of Prickly Pear Creek, located in southern Lewis and Clark County in the state of Montana in the United States. Although somewhat polluted by abandoned mines and mine tailings in its upper watershed, Tenmile Creek supplies about half the water for the city of Helena, the state capitol.
Tenmile Creek rises near the top of the Continental Divide on the east slopes of the Rocky Mountains, specifically the Lewis and Clark Range. The upper watershed consists of steep, forested mountain slopes. The lower watershed consists of the final 5 miles (8.0 km) of the stream, which travels over prairie and through residential and retail developments in and near the city of Helena, Montana before reaching its outlet at Prickly Pear Creek.Lake Helena is only 1.5 miles (2.4 km) downstream from the confluence of Tenmile Creek and Prickly Pear Creek.
Tenmile Creek is a typical mountain stream. Water flows are heaviest in late spring and early summer, when snowmelt and spring rains are heaviest.
Land uses in the upper watershed consist of outdoor recreation and timber harvesting. The town of Rimini, Montana, (37 homes) is located in the upper watershed, as is a small residential subdivision ("Landmark"), and a few scattered single-family homes and recreational cabins. The lower watershed is used primarily for farm land, ranching, and urban residential and retail development.Irrigation, primarily for farmland, draws heavily on water in Tenmile Creek, leaving the stream dry at points. But some or most of this water returns to the stream through runoff and seepage.
Hard rock mining was common in the upper watershed from about 1870 into the 1990s. The upper 13 miles (21 km) of the stream is located within the Upper Tenmile Creek National Priorities List, a federal Superfund site about 53 square miles (140 km2) in size.Arsenic, cadmium, copper, lead, nitrate, phosphorus, and zinc are released into the stream when water flows over or through mines or main tailings and reaches Tenmile Creek. At times, water in the creek contains levels of arsenic, cadmium, and lead which exceed State of Momana limits for safe human consumption, and water treatment is needed to render the water safe to drink and use. As of 2012, there were 150 abandoned or inactive mines in the upper watershed. Reclamation activity began in 1996, and was expected to be complete in 2012.
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