Lewis River Drainage, Mount St. Helens, WashingtonSummary of Drainage Basin and Channel Characteristics
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| Stream / Basin / Sub-Basin | Drainage area (at mouth) (square kilometers) |
Channel length (kilometers) |
Mean gradient (meters) |
Elevation at mouth (meters) |
Relief (meters) |
Valley trend | Channel length affected by lahars1 (percent) |
Drainage basin affected by blast2 (percent) |
Thickness of tephra deposits3 (millimeters) |
| Lewis River Basin
(affected by Mount St. Helens, May 18, 1980) |
4734 | -- | -- | -- | -- | -- | -- | -- | -- |
| Muddy River Basin
(including Muddy, Smith, Clearwater, Bean, Clear) |
351 | -- | -- | -- | -- | -- | -- | 30 | 2-1500 |
| Muddy River | -- | 29.8 | 0.024 | 315 | 2178 | NW-SE | 100 | 1 | 10-200 |
| Smith Creek | 60 | 11.9 | 0.020 | 497 | 1996 | NS-SE | 100 | 81 | 20-1500 |
| Clearwater Creek
(including Bean Creek) |
102 | -- | -- | -- | -- | -- | -- | -- | 70-800 |
| Clearwater Creek | -- | 21.7 | 0.018 | 445 | 1180 | NW-SE | 0 | 44 | 70-400 |
| Bean Creek | 21 | 11.3 | 0.027 | 523 | 1063 | NW-SE | 0 | 98 | 70-800 |
| Clear Creek | -- | -- | -- | -- | -- | -- | 0 | 0 | 2-150 |
| Pine Creek Basin | 62 | 22.0 | 0.045 | 310 | 2219 | NW-SE | 100 | 1 | 10-200 |
| Pine Creek (East Fork) | -- | -- | -- | -- | -- | -- | 100 | 1 | 10-20 |
| Pine Creek (Main Fork) | -- | -- | -- | -- | -- | -- | 80 | 1 | 10-200 |
| Pine Creek
(downstream confluence East and Main Forks) |
-- | -- | -- | -- | -- | -- | 100 | 1 | 10-15 |
| Swift Creek | 60 | 13.0 | 0.133 | 186 | 2343 | N-S | 100 | 1 | 10-200 |
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1
Extent of lahars determined from maps prepared by Janda and others, 1981;
Pierson, 1983.
2 Boundary of the blast-affected area taken from USGS 1:100,000-scale series map of Mount St. Helens and Vicinity, 1981. 3 Thickness of tephra deposits determined from isopach maps of Waitt and others, 1981. Total thickness of deposits is dominated by the May 18, 1980, directed blast (pyroclastic-density flow) and air-fall deposits. 4 The Lewis River basin has a total drainage area of 2,710 square kilometers (1,046 square miles) (Dinehart, 1986). |
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| Topinka, USGS/CVO, 1998, Modified from: Martinson, Finneran, and Topinka, 1984, USGS Open-File Report 84-614, and Martinson, Hammond, Mast, and Mango, 1986, USGS Open-File Report 85-631 | |||||||||
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