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Forest Slope Stability Project, Phase II

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Forest Slope Stability Project, Phase II 1 , .., r-; · ':::iv ' \ ! GEOLOG'/ ?J . OLYf,/lPIA. WA::JHii\iGTON 00504 FOREST SLOPE STABILITY PROJECT PHASE II VOLUME I WDOE 81-14 SEPTEMBER. 1981 Donald W. Moos Director COVER PHOTO Big Slump Mountain and Clearwater Creek, Middle Fork of the Nooksack River area V '.,,' •.' ~ FOREST SLOPE STABILITY PROJECT PHASE II for Washington State Department of Ecology WDOE 81-14 by Allen J. Fiksdal and Matthew J. Brunengo 1981 State of Washington Department of Natural Resources Division of Geology and Earth Resources Olympia, WA 98504 This study was conducted under a grant from the United States Environmental Protection Agency with funds from Section 208 of the Federal Clean Water Law (Public Law 95-217). The contents of this paper do not necessarily reflect the views and policies of EPA and WDOE, nor does mention of trade names of commercial products constitute endorsement or recommendation for use, PREFACE To assist forest managers in the identification of potential sediment sources, landforms keyed to mass wasting features and potential were ~apped in parts of five river basins in western Washington State. The study areas were chosen as representative of the different geologic, geomorphic, and climatic regions found in western Washington so that extrapolation of this study could be carried to adjoining drainages. The following report on methods used and findings is accompanied by orthophoto maps (scale 1:24,000) for the Middle Fork of the Nooksack, Green, South Fork of the Toutle, Clear­ water, and Grays Rivers. The maps have mass wasting features delineated and landforms outlined. The landforms relate to the geology and erosional processes and are rated for slope instability potential~low, moderate, or high. (The orthophoto maps that accompany this report are bound separately, Volume II.) III CONTENTS Page· Preface . .......... , ....... , .............................. , .................... , ...... , III Introduction ... , ............. , ... , ........................... , ............... ,........ 1 Mapping procedures and products ..•..•••.. , , , , , .• , • . • . • . • • • . • . • • . • • • • . • • . • • 2 Mass wasting ................................. , ............................. , . 3 Slump-earthflows ...... , ............ , ... , .. , , . 3 Debris avalanches ....•.•••..... , , , .•....••.. , •.• , . • • . • . • . • . • . • . • 4 Debris torrents . ........ , , ................ , ................. , .... , .. , , .. , 5 Stream erosion, ..... , .•..•......•...•.....• , . • . • • . • • • . • . • . • . • . • . • 6 Rock slide . ..................................................... , . 6 Rockfall ... , ....... ,.................................................... 6 Snow avalanche chutes, .••• ,, .....•.....•...•••.•...•..•.•...........• ,, 7 Road sidecast failure ..•.•.. , ... , ...•...•...•.•.. , ••. , ..•....••. , . • • • • • . 7 Roadcut backslope failure. , ••..•.•.••. , .•.• , ...•......••••.....•....••. , 7 Summary............................... ,....................................... 8 Landforms ... , ............. , , ................ , , .................... , , .. , .. , ... , ... , . , . 8 Alluvial-glacial terrain •. ,.,, .........•...••.•.. , .........•...............•..• ,. '9 Bedrock terrain .. , , .•....•........ , ......•• , , , , , • . • . • . • • . • . • . • . • • • . 10 Landslide terrain .•.... , . , ••....•.•...•...•. , .............. , •.•.•...•...• , • • . • 13 North Cascades-Middle Fork of the Nooksack River area •..•••. , ••••.•.• , •• , .•• , • , , , • , 14 Alluvial-glacial terrain .. ,., ••• , ...•........••.•.....•. ,., ••.. ,, ••.• , ..•.••.. ,.. 16 Floodplains ...... , . , .. , ........... , .... , ........................ , . 16 Terraces ............. , ........ , ...................... , .. , . , ......... , , . 16 Valley benches . , , • , ..•...••.......••••.....•.• , ..••••••.• , •• , , •• , .••. , , 16 Alluvial fans and cones ..•••.......••.•. , . • . • • . • . • . • . • . • • . 16 Continental glacial deposits .....•.•••....• , , .•. , • • . • . • . • . • • . • . • • 17 Glaciated valleys ....••..............••. , •...••.• , . • . • . • • • . • . • • . • 17 Cirque floors . ............................. , ............. , , . , . 17 Cirque headwalls •.•.....•••• , .•....•...• , • . • • . • • . • . • . • • . • 17 Mature bedrock terrain • • • . • . • . • . • • • • . • . • • . • . • . • . • . • . • . 17 Moderate slopes .... , ............................................... , . 17 Steep slopes . .......... , ....... , , ............................ , ........ , . 18 Twin Sisters foothills ..•....•......•..••..•.•...• , •.••.. , ...•.. , ...• , . 18 Twin Sisters alpine zone ...... , .. , ..•..•..• , ..• , .....•.•...••.•..... , .. , 18 Sandstone-shale terrain ••..•.•. , , •.•... , ..... , .•.•.•.• , , •... , .• , •.••......•... , . 18 Dip slopes. , ....... , . , ... , , . 19 Scarp slopes, •.•.•...••.••••... , , ••....••••.•..•..••....•.•... , .• , , , .. , • 19 Ribbed slopes ..... , ........................................ , ........... 20 Undercut slopes • , , , •......•.....••.••.. , .•... , . , .•.•....••..•...•.• , • • . 20 V Page North Cascades-Middle Fork of the Nooksack River area--Continued Landslide terrain .......•.... , • . • . • . • . • . • 20 Slump landforms ........ ,, ..• ,,,.,., .. , ....... ,,., •.. ,,,., ..•...•....... 20 Incised valleys , , • , .....• , , ... , , , ... , , , •.. , . , •...•...•.. , ....... , , , • • . • . 21 General instability and hazards , ... , , ..••. , ... , , • . • . • . • . • . 23 Central Cascades-Green River area ..••..............•.....•...•.•.•.........••.•.••. 23 Alluvial-glacial terrain .•...•....•...•.•.. , •..... , .....•.••..•..••.............. 25 Floodplains .•.•.•...•.... , ..•......•.... , ....... , . • • . • . • . 25 Tributary valleys, . • . • . • . • • . • . • . • . • . • • . • 25 Terraces , ........•.....•..•..... , ....•• , ...• , , ....••. , . • . 25 Benches ...•...•..•..••.•...•. , •..•.•... , ..•.•...•... , .. ,., ......•.....• 26 Alluvial fans .....•..••...•.................... , • , ........•.•.. , . • 26 Glaciated valleys .•............••....• , .....•.....•.• , • . • . • . 26 Cirque floors .......•....... , •...•. , ... , .........•.. , . • . • . 26 Headwalls ..•....... , ... , •.. , ... , .. , ......•.. , ..•.••....•...•... , . • • . 26 Volcaniclastic terrain. ...•... , .. , ...•...•..•...•..... , .....• , ....•..........•... 26 Flat bedding attitudes ....•.••.... , ....••..............•.. , . • . • 26 Dip slopes ••...•.•...•••.•......•••....... , ...... , ....•••.•....... , .. ,.. 26 Scarp slopes •.......• , . , ....•. , . , ..••...• , ...••••. , • . • . • . • . • 27 Eagle Gorge terrain •.....•.•.....•... ,, .....•......... , ..... , ............•..... 27 Dip slopes .......• , ... , •.•.•....••. , ......•.....•...... , , . • • . 27 Scarp slopes ....•.... , ....•.............•...•. ,, .. ,..................... 28 Snow Creek terrain. , .•.......•...... , ..•. , .•.....•...•.•... , .•..•.... , . • • . 28 Snow Creek slopes ................ , ......••. , ....•...................... 28 Intrusive terrain. .... , •...•.... , ... ,.,, ..... ,, .. , ..... , ..•.........•....... , .. , 28 Intrusive terrain. ............ , ..... , •...•............. ,, ..••... ,........ 28 Landslide terrain • , .. , .. , ...•.. , ...••.... , . • . • . • . • . • . • . 29 Slumps on volcaniclastic rocks ....•.•. , . • . • . • . • 29 Slumps in Eagle Gorge terrain .............•.....••.•.....•...•.•. , ....• 29 Incised valleys •.•......•.....•....... , .•....... , . • . • . • . • . • 29 General instability and hazards . • . • . • • . • • . • . • • • 29 Southern Cascades-South Fork of the Toutle River area ..•......•....... , .•..•.... , . 31 Alluvial-glacial terrain ......................................•..•........ ,...... 34 Mudflow-alluvial valley .. , ..............•...•.• , . , ..• , ...•.... , ... , . • 34 Older mudflows or pyroclastic flows ............ , ... , .... , ... , .. , ... , . 34 Terraces and benches.. , ..... , •. , .. , ... , ........ , ............. , ..... , . 34 Glaciated valleys ...•....... , •. , .•.......•.•....•.. , ... , . , ... , .. , . , , . 34 Glacial headwalls-valley sides ..•.. , .•• , . , . , .•.. , ..• , . , . , . , .. , , ... , . , ... , 35 VI Page Southern Cascades-South Fork of the Toutle River area--Continued Dissected volcanic terrain, .......•.•••••••...•.••••••..•• , •.•. , , •.• , , .• , ••• , • • • 35 Dissected slopes •.•..•.••....•.•.•.•...•••.•.• , , ••..••..•••..•••••. , •• , . 35 Dip slopes, , , . , . , , , • , , • , . , , • , , . , , , , • , , , , •. , • , , • , , , .•. , , , .••• , , , , , , . , • , • • 35 Scarp slopes.,., •. ,,.,., ••• ,,,,,,.,.,,,,,,.,,,,,,,,.,,,.,,,,, .••• ,., .•• , 35 Ohanapecosh terrain . 36 Ohanapecosh dip slopes, • , , , • , , , . , • , , , , .• , , , , • , . , , , , , • , • , •.• , , .•. , , , • , , • 36 Ohanapecosh scarp slopes •. , . , • , • , , , , , , , , , • , , •••••. , , , , , , • , , , , , .•• , , •. , , 36 Bedrock terrain. , , , , • , •.•. , , • , , , , , , • , , .. , . , •• , • , , , , , , , , , , , , • , , • , . , •. , , ... , • . • • . 36 Lava flow ....... , ........ ,............................................... 37 Goat mountain intrusion •• , , , , , •. , , , , , , , , , , , , , , , , , , •• , • , • , • , , •• , , • • • • . • . • 37 Landslide terrain , , • , , ..• , . , , • , , . , , . , , , . , , • , , • , , , . , , , .• , • , ••• , •• , . , ..• , . • . • • . 37 Incised valleys ..••• , , .•..• , • , , , , ••••.. , .. , •.. , , ... , ..• , , •. ·, , • . • . • • • • 37 Eroding
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