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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 valley sides , , , , •• , , , • , ••. , , •• , , • , , , , • , , .• , , , • , , •••.•..•••. , . . . • 38 General instability and hazards , , •.. , . , , , , , , . , , • , , • , •. , , , , , . , , , •..•... , • , • . • . . . 39 Olympic Peninsula-Clearwater River area, , , , , .• , • , , , , •.•.•.•. , .• , •••• , .• , , , , , , • • . • . • . • 39 Alluvial-glacial terrain., •.•• ,., .. ,., •. ,., ..•• ,, •.. ,,,, •••• ,, .•• , •••... ,,., .••.• 42 Floodplains ...... , ...... , ...... 42 Terraces ...... , . , ...... 42 Alluvial fans .• , •..•• , . , •. , , • , .. , , , , •• , . , .•.. , , , , ..•.• , , . • • . • . . • . . • • • . • • • 42 Glaciated valleys. , , , . , , •.•• , . , .•.....•...... , ..•... , • . . . • • • . • . • . • • . • • • . . 42 Glacial slopes ....••.•...• , . , , , , , . , .• , , , , . , • , , . , , •••••... , . . • . . • . • . . • . . . . 42 Glacial terraces ••...... , .• , .... , ..•. , •..••.... ,, .... , ... ,...... 43 Bedrock terrain ..••••..• , .. , , , ••• , . , . , •. , .•. , ...•.....•••.•. , .. , • . . . . . • . . . • • • • 43 Ridge tops .•••••.•.••. , • , . , • , •.•.•• , , • , •.•• , , , .. , .•. , ••••.•.•. , . . . • • • . • 43 Dissected gentle slopes ...... •. , .•...... •..•...... •..• , . , 43 Dissected slopes , ..•••..•...... •.....••.. , ....••.•.•. , • , • , , ..• , , , , , , • 43 Landslide terrain •..•.•...•...•...•.•....•...••• , ...... ••...• , •.•.•.•.. , , , , , , , • 44 Incised valleys , ..•••....•...... •. , . • • . . . • • . . • • • • • • . . • . . . • • . . . . . • • • . . . . • 44 General instability and hazards •...... •..•••.••..••....•...• , ••• , . , • , ...... 44 Willapa Hills-Grays River area .•.•.•.••.•..•.•. , ••...••••.....•.. , . , ...... ••• , .. , • • 45 Alluvial-glacial terrain .....•...•. , .....•.•...•.•....•..•.•.....•.•.•.•••...• ,., 47 Floodplains .. ,., ..•.•. ,., .. , .• ,., ... ,,, .••.• , ....•.•.... , ...... , ...•.•. 47 Tributary valleys. , , , , , , . , ..... , , ..• , .•. , ••. , .. , , , , , , , .... , .••..•••. , • . • 48 Terraces-benches •...•. , ..... , , ... , , ••. , .•• , , , ...•.••. , .. , .•.. , .• , ••.. , • 48 Crescent terrain ...... •.•.•..•...... •..••.. ,.,, .••.. ,,.,., .. ,., .. , ... ,. 48 Dissected topography ..•...... , •..•. , ..... , , , . , •.••. , , . , , . , , , . , . , •.. , , • 48 Scarp slopes, •••• , , • , .. , .••.. , .•...•...•. , ....••••••.•...... • , , .•..••.• , 48

VII Page Willapa Hills-Grays River area-Continued Bedded sedimentary terrain. , • , , ••..•.•• , ••..•.•• , , .•..•• , • , ... , .••..•. , . . . • • • • 49 Moderate slopes, ••..•. , , •••.• , ••.. , •.• , , •• , ..• , , •.••..•.•.•••••.. , .•• , • , 49 Scarp slopes . .... , •1• •••••••••••• , ••••••••••••••••• , •• , •••••• , •••••••• : • • 50 Lincoln Creek topography, •. , • , ••.••.••••.••..•••• , • , , •• , , , • , • , , , , , • , •• , 50 Gentle topography, .•• , . , •• , • , , . • . • . • . • . • . . . • . • • • • • • . • • • • . . . . • • . . • • • • • • • 50 Goble terrain ...... , .... , ...... , ...... 50 Slump topography ...... , ...... , ...... , ...... , ...... 50 Other slopes, ...... , ...... 51 Landslide terrain ...... 51 Slump topography •• , • , , , .. , . , ...... •.•••.••.•.•.. , • , .•... , ••.•.•... , , , • 51 Flow topography, .. ~, ...... 51 Incised valleys .... , ...... 52 General instability and hazards , , .••.• , • , ••• , , . , , •••... , , ..•.•. , .•.•....•.•.• , , 52 Summary .... , ...... ,., ...... 54 References cited, •.. , .....••.•.••••••.••.•••. ,., ..•••..•..•.... , .. ,,, ••••..•.••• , •.•.• 56 Appendix I-Glossary .••.•..••• , , , ••.•..••••••. , ...•••.. , .• , • , •. , •.•• , •...... , . , •• , • • • 60 Appendix II-Geologic time scale .•.••..••.•..•.•••.••••. ,., ••..•••..••..•••.••...•.•••. 62

VIII ILLUSTRATIONS

Page Figure 1. Index map of study area . , , , , . , . , , , , , , • , , , , , , , , , , , , , , , , , , , , , . , , , • , , , , , , , , 2 2. Four types of mass wasting features, , , , • , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 3 3. Slump-earthflow, Big Slump Mountain , , , , , , , , , , , , , , • , , , , , , , , , , , , , , , , , , , , , • 4 4. Debris avalanche, Clearwater basin ..•.••• , • , , , , , • , , . , • , , , , , , , , , . , , , , , , , , , 4 5. Debris torrent scars on 1st-order tributaries, and a channel scoured by debris torrent , , . , , , , , , . , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , • • • · • 5 6. Rockslide ...... , . . . . 6 7. Road failure ...... , ... , ..... ,. 7 8. Diagrammatic sketches of bedrock dip and hill slope relationships , , , , , , , , , 11 9, Diagrammatic sketch of dip and scarp slopes , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 12 10. Middle Fork of the Nooksack River index map,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 15 11. Stereo pair of Clearwater Creek area , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 16 12. Mature bedrock terrain, Grouse Ridge , , , , , , , , , , • , , • , , , • • , , , , , · , , , , , , • , , • , 18 13. Twin Sisters Mountain alpine topography,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 18 14. Slump-earthflow on dip slopes north of Canyon Lake, , , , , , , , , , , , , , , , , , , , , , 19 15. Ribbed slopes , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . , , , , , , , , , , • , , , , , , , , , , , , , , 20 16. Large slump-earthflows, Middle Fork of the Nooksack River basin • , • , • , , • , 20 17. Green River index map , . , . , .•. , ... , , ....••.•. , •. , • , .. , . , •.. , . , •.... , , ••• , 24 18. Dip slope in volcaniclastic terrain .•. ,., .•.•• ,., ..••.••.• , .• ,., •.•..•. ,,.,. 27 19. Eagle Gorge dip slopes ..•.•• , , •.. , , , ..• , •. , ...... , . , .. , .. , •.•• , .• , •••• , , 27 20. Snow Creek terrain north of Smay Creek , .. , . , ...... •• , •.. , , .• , • , , .. , • , . 28 21. Intrusive terrain, Rooster Comb ...•.•••.. , ...•.•• , , .•••.••.•.. , •.•• , •.•• , 29 22. South Fork of the Toutle River index map ..•• , ••... , . , ••.• , .• , .• , •• , . , • , . 33 . 23. Bouldery mudflow surface of the South Fork of the Toutle River valley . , • 34 24. Gentle dip slopes on south side of South Fork of the Toutle River valley. , 35 25. Stereo pair of scarp slopes east of Signal Peak , .•••.•.• , ... , . , , • , , , . , • , , , 36 26. Dip and scarp slopes, Goat Mountain intrusion •••••• , .•..•••.••. , •. , ...•. , 36 27. Clearwater River index map, ...••.•..• , . , ..... , .••••. , •.••.•• , ••... , • , •. , . 40 28. U-shaped glacial valley in upper Clearwater River ...•.•.••••.••.•• , .••• , , 42 29. Clearwater River valley east of Nancy Creek, .•..••..••.••.• , .•..•.. ,., ••• 43 30. Grays River index map •• , ...••...•••• , .•. , ••.•.•••...... ••.... ,.,., •..• ,, 47 31. Crescent dissected terrain. , ....• , . , ...•••• , .•.. , ••• , •. , , •...• , , , , • , , , , , , , 48 32. Stereo pair of bedded sedimentary terrain ..•.••....•••. , ..•• , •.. , .•• , , • , • 49 33. Stereo pair of Goble slump topography •..••...•.•• , •.••. , •. , ...••...... • , . 51 34. Main fork Grays River incised into Goble volcanics. , •. , .• , • , .• , .••••.. , . , , 52

IX ILLUSTRATIONS-Continued

Landform maps and mass wasting maps numbers 1 through 72 are bound separately as volume 2.

TABLES Page Table 1. Generalized instability of Middle Fork of the Nooksack River landforms .•... 21 2. Types of hazards associated with Middle Fork of the Nooksack River landforms . . . • . . . • • . • . . . . • . . • ...... • . • . • ...... • • . . . • • . . . . . 22 3. Generalized instability of Green River landforms . . . • ...... • ...... 30 4. Types of hazards associated with Green River landforms ....•.•.•...... 31 5. Generalized instability of South Fork of the Toutle River area landforms .•. 37 6. Types of hazards associated with the South Fork of the Toutle River landforms ...... • ...... • • . • • • . . • . . • . • • ...... • • . . . . . • • . • • . 38 7. Generalized instability of Clearwater River landforms. • . . • ...... • . • . . • . • 44 8. Types of .hazards associated with the Clearwater River landforms. . • . . • . . . . . 45 9. Generalized instability of Grays River landforms • . . . . • . . . . • ...... • . • • • • 53 10. Types of hazards associated with Grays River landforms • . . . . • • . . . . • ...... • 54

X FOREST SLOPE STABILITY PROJECT PHASE II

by Allen J. Fiksdal and Matthew J. Brunengo

INTRODUCTION

There is a potential for water quality private organizations concerned with forest degradation resulting from mass wasting slope problems and geologic and forestry caused by forest practices in the steep expertise was formed and a field team set up. forested areas of western l~ashington. To A reconnaissance of western Washington was assist forest managers and resource planners completed in the first phase of this study alleviate this potential water degradation and was published in 1980 as DOE Technical problem the Department of Natural Resources Report 80-2 (Fiksdal and Brunengo). (DNR) conducted a study to evaluate slope In Phase I, mass wasting features were stability on forested terrain in western identified on 1:62,500 air photos and Washington. The study developed informa~ion published on 1:250,000 scale maps of western for identification of watersheds, which exhi­ Washington. Also five river drainages were bit a greater-than-average susceptibility chosen as representative study basins of to mass wasting, and mapped slope stability the different geomorphic regions in western in selected watersheds, v,hich are thought Washington fcir the final (Phase II) part of to be representative of the different geolo­ the Forest Slope Stability Study. gic, geographic, and climatic conditions in The five drainages chosen were: (1) the western Washington. The project was funded Middle Fork of the Nooksack River, repre­ through the Washington State Department of senting the North Cascade Mountains; (2) the Ecology (DOE) and is called the DOE Forest Green River, representing the central Slope Stability Project. Cascades; (3) the North Fork of the Toutle In 1979 the DOE contracted with DNR to River, representing the southern Cascades; undertake a forest slope stability study (4) the Clearwater River, representing the following a pilot slope stability study of Olympic Mountain Peninsula; and (5) the the Deschutes River area conducted in 1978 Grays River, representing the Willapa Hills by DNR (Thorsen and Othberg). The pilot region (figure 1). project proposed a method for studying other A change in the Toutle River study area drainages in western 1Jashington that was was caused by the eruption of Mount St. generally adopted for this study. An advi­ Helens, in May 1980. The blast and subse­ sory group representing state, federal, and quent debris and mudflows in the North 2 Forest Slope Stability Project

124° 122• In order to accomplish this, we mapped 4 erosional and depositional features on air· photographs and field checked these features • along with the geologic and hydrologic con­ ditions. The basins were then divided into 48° landform or terrain units, within which the geologic and mass movement characteristics are broadly uniform. Color aerial photographs at 1:24,000

GR££N RIVER scale were used in Phase II. Photo coverage at this scale was already available for each drainage facilitating our study. The photos were taken under various DNR contracts in 1975, 1976, 1978, and 1980. In addition, STUDY AREAS IN 1978 black-and-white photos (1:12,000 scale) WESTERN WASHINGTON of the Green River basin were also examined, in order to evaluate the effects of a Figure 1.-lndex map of study area. damaging storm the previous winter, and 1979 black-and-white (1:12,000) photos of the Toutle drainage significantly modified the Clearwater basin were used to update the drainage and eliminated access to the area 1975 color air photos. so the study area was changed to the South Reconnaissance field work was carried Toutle drainage, which has a similar geo­ out between October 1979 and February 1981. morphic setting. The primary purpose of this work was to The DOE Forest Slope Stability Study verify the accuracy of air photo mapping. Phase II followed the Pilot Project format We found that photo analysis at a scale of by using a landscape approach to its slope 1:24,000 or larger is suitable for identi­ stability mapping. Mass wasting features fication of most existing mass movement were mapped and keyed to landforms. Land­ features. forms were then mapped in the study drain­ The secondary goal of the field work ages, providing a tool that forest land was to understand the geologic and hydro­ managers can use to identify other areas of logic conditions governing mass failure. similar mass wasting processes for resource It proved impractical to do complete measure­ planning. ments and stability analysis on even a small proportion of the failures. However, we did observe the earth materials involved in MAPPING PROCEDURES AND PRODUCTS representative slides, their weathering condition and mechanical state, the struc­ The goal of Phase II was to identify the tural attitudes, and the downhill effects geologic conditions controlling mass wasting of the failures. We also looked for evi­ within each study basin, and to integrate dence of stream undercutting, ground-water them into a landform analysis, similar to seepage, or failure of road drainage that in the Slope Stability Pilot Project. systems. In other words, we attempted to Mass Wasting 3 relate the type and frequency of mass move- The mass wasting map units are described ment to the physical conditions in an area. below; for further discussion see Burroughs. This study covered all or parts of and others (1976), Swanston and Swanson forty townships and a mass wasting and {1976), Rib and Liang (1978), Rickert a.nd landform map was produced for each. The others (1978), and Varnes {1978). maps are on 1:24,000 scale orthophoto ma~s, Slump-earthflows {E*}: Landslides in­ each covering one township. The orthophoto volving slump (movement downward and out­ maps we feel are a good vehicle because ward, combined with backward rotation they are available for a majority of areas along an arcuate failure surface) and earth­ of Washingt'on State, are easily obtainable, flow (slow flowage of broken slump debris and are already used by most forest managers. downslope) occur in many kinds of materials, The orthophoto maps are also in easily but most often in unconsolidated sediment usable scale (1:24,000) and have not only and weathered rock. They are usually deep a photo image but section lines, geographic seated and involve rock, regolith, and soil names, and contours on each image. (figures 2 and.3). Regolith includes The following text describes and defines the mass movement and landforms found in the study areas and their relationship to slope stability problems and/or potential.

MASS WASTING

Very slow to rapid The mass wasting features observed and mapped on the air photos were slump-earth­ flows, planar rockslides, debris avalanches, debris torrents, failures due to stream incision, road and backslope failures, and snow avalanche chutes. These erosional features were mapped onto photo overlays and transferred to the orthophoto maps. Very rapid to extremely rapid Large features were mapped at true scale, with arrows indicating direction of move­ ment where appropriate, while those smaller than approximately 100 m2 (1100 ft2) in size are identified by a symbol on the maps. We also identified large rock outcrops or Extremely rapid free faces, and depositional features such Oipstope control by bedding ptanes as channel sedimentation, debris torrent Rockslide lobes, and fresh talus on the photos, but VERY SLOW TO did not transfer these to the maps. EXTREMELY RAPID

* Symbol used on maps. Figure 2.-Four types of mass wasting features. Forest Slope Stability Project

Debris avalanches (A): Debris avalanches occur when a thin layer of soil, fill, or weathered material fails and moves rapidly downslope (figures 2 and 4). This type

Figure 11.-Debris avalanche on steep hillside, Clearwater Basin.

of mass movement commonly takes place on steep slopes greater than about 30°, often in depressions where concentrations of sub­ Figure 3.-Slump-earthflow, Big Slump Mountain. surface water cause reduction of shear weathered rock, unconsolidated sediment strength. The failure surface is usually (alluvium, colluvium, glacial deposits, a well-defined interface between weathered etc.), and soil. In this report, "soil" regolith and relatively stronger material, refers to earth material that has been so such as bedrock or till. Road fill and modified by physical, chemical, and biologi- sidecast are also susceptible to failure by cal processes that it can support plant debris avalanching. growth (pedological soil). On the air photos, debris avalanches S1ump-earthflows were identified on the were identified by narrow streaks of light photos by arcuate headscarps (often still color where soil and vegetation had been partially unvegetated), large central stripped away. They commonly begin at V­ depressions, hummocky topography, and dis­ shaped headwalls (as opposed to the U­ rupted drainage or ponding. Identification shaped headscarps of slumps). If the debris was difficult if the features were indistinct, becomes channelized and mixed with water, or if small slumps were hidden by trees. debris torrents form. The contribution of slump-earthflow to Because debris avalanches are shallow stream sedimentation varies with size, failures, they are strongly affected by proximity to streams, and degree of activity. forest management activities which remove Fractured and weathered debris is weak and support by undercutting (as in roadcuts), subject to reactivation by stream incision by destroying root strength (by logging), or and management activities, such as road by altering soil moisture content and move­ cutting and drainage changes. ment (by logging, compaction, disruption of Mass Wasting 5 drainage). Thus, debris avalanches and the least two 2nd-order tributaries and 4th­ often-resultant debris torrents are the mass order streams have at least two 3rd-order movement types most susceptible to aggrava­ tributaries. tion by road building and logging. The Debris torrents (D): When debris from mapped distribution of debris avalanches is a slump or avalanche reaches a stream, it biased towards roaded and logged areas can mix with the water to become a thick because of this causal relationship, as well slurry which flows down the channel. This as better exposure in clearcut areas. On fast-moving slurry or debris torrent the other hand, debris avalanche scars usually scours the channel to bedrock. When usually revegetate within a few years. it finally stops (usually where it flows Therefore, the mapped avalanches are those into a larger stream with a lower gradient), which had occurred within about 5 to 10 years it can deposit a mass of rock, soil, and before any photo was taken. organic debris several orders of magnitude Debris avalanches and torrents are a greater in volume than the initial failure. major contributor of sediment to streams on Debris torrents are identified on air forest lands. Avalanches generally deposit photos by the light color of the channels, debris directly into 1st- and 2nd-order where alluvium and vegetation have been stream channels, First-order streams are stripped away and bedrock exposed, and by those having no tributaries. Second-order the deposition of sediment in lower channel streams have only 1st-order streams as reaches (figure 5). tributaries. Third-order streams have at As with debris avalanches, debris

Figure 5.-Debris torrent scars on 1st-order tributaries (left), and a channel scoured by a debris torrent showing deposition of debri_s behind a road (right). 6 Forest Slope Stability Project torrents are normally concentrated in logged taken whenever activities must occur in areas because roading and logging increase erosive streamside zones. this kind of mass movement activity. Also, Rockslide (R}: In areas of competent, the mapped torrents are mainly a 5- to 10- well-bedded and jointed rocks, large slabs year sample; most channels are subject to can become detached and slide along planar this kind of mass movement, at varying inter­ failure surfaces, especially when the beds vals of time, as part of the natural process or joints dip parallel to surface slope at of erosion, but the scars usually heal within an angle of about 35° or more. Such rock­ a few years. Debris torrents scour vegeta­ slides are identified in the photos by tion and sediment from channels, leaving large patches of bare, smooth bedrock. little food or hiding or spawning areas for Most are located in the interbedded sand­ invertebrates and fish. Deposition of stones and shales of the Middle Nooksack debris often dams the stream, hindering fish basin (figures 2 and 6). passage, and reworking of the debris deposits by water can cause sedimentation downstream. Stream erosion (St): There is active undercutting of hill slopes along many streams. This may take the form of arcuate slumps in terrace scarps or bedrock slopes, or many small avalanche scars located along stream cuts. In either case, active mass erosion and removal of debris are taking place, and there is the potential for further undercutting. These areas have been mapped as stream erosion, and occur in both indivi­ Figure 6.-Rockslide with failure along a shale dual sites and in more extensive erosion interbed. zones. It should be understood that almost all Rockslides present a particular hazard of the 1st- and 2nd-order tributary channels to forest users because of the suddenness exhibit stream incision. Therefore, stream­ and rapidity with which they can move. The bank erosion zones are mapped only along the effects on stream quality depend upon the larger streams, except where scars are size of the failure, the size of the broken fresh or large enough to be identified and material and access to streams. Many of mapped separately. the rockslides occur high on the hillslopes, Stream erosion zones are the areas of and do not seriously affect the major streams most active natural mass wasting. In addi­ below. tion, the proximity to the stream means that Rockfall (Rf): l,Jhere large areas of the debris will have a direct impact on bare bedrock have been exposed, rockfall water and stream quality, with no opportu­ may occur (figure 2). These outcrops (or nity to be delayed or filtered. Timber free faces) do not have the protective harvest activities can increase runoff cover of soil and vegetation, and are sub­ rates, and thus increase stream erosion ject to physical weathering and direct rates. Therefore, special care should be fluvial and avalanche erosion. Rockfalls Mass Wasting 7 are easily identified on air photos by and poorly drained, and usually fail in the free faces with piles of talus at their same way as debris avalanches. Sidecast bases. is especially sensitive when it covers small Hazards associated with free faces springs or seeps, and fills are most vul­ include rapid rockfall and rock and snow nerable where they cross small headwater avalanching, which may deliver a periodi:c streams. Stream channels are likely to be but continuing supply of coarse sediment disrupted when failure occurs at these to streams. locations. Sidecast and fill failures were Snow avalanche chutes (C): Chutes mapped on the photos where there was an heading on high, subalpine to alpine ridges excessive amount of sediment moving downhill were mapped as avalanche chutes if they con­ from the road, or where loss of part of the tained anomalous vegetation patterns roadbed could be seen. (younger trees, deciduous or brush species), Sidecast failures constitute the most but did not exhibit the degree of scour ~ignificant mass movement process related indicative of debris torrents. The to forest management practices (see U.S. mechanism of failure is similar to that of Environmental Protection Agency, 1975, for debris avalanches, except that the layer references). They act much like debris that becomes detached is made of snow and avalanches, and trigger a large proportion ice, instead of regolith. Some soil and of debris torrents, with resultant impact vegetation are usually entrained by an on stream quality. The susceptibility of avalanche, but most of the mass is snow. roads to failure depends partially on the Therefore, there is little sedimentation rock type and soil conditions, slope, and impact from this type of failure, though it hydrology, but these conditions can be does present a hazard to people, equipment, aggravated or ameliorated by the road and facilities. location, design, construction, and mainte­ Road sidecast failure (X): Failure and nance. movement of sidecast material and the road­ Roadcut backslope failure(•): Removal bed is extremely common in steep forest of part of a hillslide for a road can criti­ lands (figure 7). Sidecast and fills are cally undercut the slope, causing failure. conmonly oversteepened," poorly compacted, The movement may be similar to that of a debris avalanche, with only a shallow layer of soil involved, or it may occur as a rotational or planar bedrock slide. Any slides located directly above roadcuts were mapped as roadcut failures, although undercutting may not have been the cause of some. Also, it was sometimes difficult to differentiate between cut failures and intentional cuts into the hill­ side for fill material and road rock. As a rule, roadcut failures cause less. stream sedimentation than sidecast or Figure 7 .-Road backslope and sidecast failure. fill failures do, since the road itself 8 Forest Slope Stability Project usually catches most of the debris. How­ LANDFORMS ever, this causes some problems for road users if the road is blocked, as often Two approaches have been used to happens. At the least, small cut failures classify slope instability potential at the and dry ravel can disrupt the road drainage regional scale, the parametric approach and system, which may cause saturation of the the landform approach. In the parametric roadbed and more serious failure. Rain­ approach the factors that may seem to have splash erosion of the bank-cut, roadbed, the greatest effect on mass movement, such and sidecast also contribute sediment to as rock type, slope angle, and aspect are streams after every storm. given a judgmental empirical value. These factors and their assigned values are put SUMMARY into a stability formula for a rating of potential instability. Rickert and others Several problems and biases in the air­ (1978) used lithology, soils, and slope photo mapping should be mentioned. Accuracy angle as base parameters in their study of is affected by several factors, such as the a basin in southern Oregon. Nilson and age and size of a feature, and vegetation others (1979) incorporated slope steepness, cover. Debris failures may become nearly strength of geologic units, and distri­ unrecognizable after several years. The bution of old landslides to classify slope ability to distinguish smaller features stability in the San Francisco Bay region. often depends on the scale of the photos or These projects had the excellent technical degree of timber removal. Roading and and professional support (good geologic and logging both expose pre-existing slides and soils maps, ,special infrared aerial photo­ cause ne1-1 ones, so more features are identi­ graphy, computerized slope maps, etc.) fied in cleared areas. In addition, a large necessary for the parametric method. proportion of the failures in a basin occur The rationale behind the landform during large rainstorms and rain-on-snow approach, summarized by Rib and Liang (1978), events (recurrence interval 10 to 20 years?). is that recognition of landforms, and appre­ We observed the effects of such a storm on ciation of their genetic aspects, makes it the 1978 photos of the Green River basin. possible to, estimate their potential for In summary, the mass movement maps are mass movement. Thorsen and Othberg (1978) fairly accurate representations of large used the landform approach in the Slope landslides of any age up to about 15,000 Stability Pilot Project in the Deschutes years, of small recent landslides and earth­ River basin. They divided their study area flows in cleared areas, and of debris ava­ into categories based on readily identifiable lanches and torrents up to 5 to 10 years old. landforms, and then described the mass move­ The maps serve to flag areas that are espec­ ment activity and potential for each unit. ially susceptible to different kinds of \fo used the l andform approach (with failure, and they aid in the preparation of parametric overtones) in this project. The 1andform maps. scope of our study, the lack of complete Alluvial-Glacial Terrain 9 geologic, soils, and slope maps, and limi- ALLUVIAL-GLACIAL TERRAIN tations on time and manpower prevented us from using the parametric approach. We Alluvial terrain is usually combined feel that since a landform is the product with glacial terrain in this study, since of a certain set of processes acting on both consist mostly of low-gradient, largely materials over some period of time, the depositional landforms. Alluvial terrain landforms embody all of the factors relating consists of floodplains, tributary valleys, to instability, such as rock type and terraces, higher valley benches, and structure, soil development, hydrologic alluvial fans. These landforms have been conditions, geomorphic history, and current shaped mostly by a combination of fluvial erosional processes. Distinct landforms erosion and deposition. Mudflows, mass are easier to identify, on air photos and movement, and glacial processes have helped in the field, than zones defined on the shape or modify some of these landscapes. basis of parameters. This makes them easier Since they generally have low slope gradients, to recognize (even by people with little alluvial landforms are less subject to mass training in geology or geomorphology), and movement than most other terrains, except in easier to apply to regions outside our areas of undercutting or incision by streams. study basins. Glacial terrain consists of continental Our classification system, therefore, and alpine glacial landforms. Continental is based on landforms, which relate back glacial processes left hummocky surfaces of to lithology, structure, geomorphic history, till (silt, sand, and gravel compacted and the types and distribution of mass move­ beneath the ice) and smoother alluvial sur­ ment processes. The landforms are grouped faces of outwash (loose sand and gravel into terrains, or collections of landforms deposited by glacial melt-water streams) made of similar materials and shaped by along the northern Cascade Mountain front. similar processes. The terrains may be Alpine glacial processes left U-shaped named after a set of processes, a distinc­ valleys with oversteepened sides, cirques tive form, or a rock type, but, like the with steep headwalls in the eroded higher landform units, they designate areas in elevations, and flat wide valley floors in which the mass movement processes and poten­ the depositional areas of the lower valleys. tial are broadly uniform. In the Clearwater basin, terraces of fine Due to the differences in geology and lacustrine sediments formed in a glacial­ geomorphology between the different regions dammed lake. of western Washington, the classifications The susceptibility of glacial land­ used in the five study basins are somewhat forms to mass movement is variable. Glacial different. However, we recognize five valley bottoms, cirque floors, and low­ basic types of terrains: alluvial land­ relief continental deposits are fairly forms, glacial landforms, landforms shaped stable, while the oversteepened cirque head­ on more or less homogeneous rock types, walls and valley sides are usually unstable. landforms shaped on strongly bedded rock Since they have poor permeability, till and types, and landforms shaped predominantly lake sediments cause perching and concen­ by active erosional processes. tration of ground water, which reduces the 10 Forest Slope Stability Project

strength cf rock and regolith (weathered, and undercutting by rivers and incision by unconsolidated material) masses. On the tributary streams may be undercutting and other hand, most outwash is permeable, and destabilizing the hill slopes. Many of these therefore much less prone to ground water zones are included in t~e incised valley concentration which usually results in landform units. The other instability zones unstable conditions. Steep slopes and im­ are the steep areas below the ridges where permeable materials should be of most con­ tributary streams are eroding headward and cern in the glacial terrain. oversteepening the headwalls at the heads of the drainages. Concentration of ground BEDROCK TERRAIN water in these small swales or hollows at the heads of drainages makes them more Most of the land in our study areas susceptible to slope failure, especially has been shaped by a combination of weathering, debris avalanche, than are the spurs and fluvial erosion, and mass wasting acting on straight slopes. The hollows are too small bedrock. These areas can be divided into and numerous to be mapped as separate land­ two general terrain types: those formed on forms at this scale, but they must be con­ rocks in which bedding is not present, or not sidered potentially unstable parts of most an important factor; and those formed on bedrock landforms, especially those that strongly bedded rocks, which greatly in­ are extensively dissected. fluence the erosion and mass wasting charac­ A special type of massive rock is the teristics of a hill slope. igneous intrusion, made of rocks that soli­ The first group of rock types includes dified from magma at depth but now have been massive sediments, strongly sheared or exposed by erosion. These rocks are com­ foliated rocks, granular rocks (such as monly hard, strong, and jointed, and resis­ igneous intrusions), very poorly bedded tant to erosion. They usually form plug­ volcanics, etc. Bedding and jointing are like masses of bare or nearly bare bedrock, commonly absent and there are no consistently surrounded by piles of talus. The Twin preferred failure planes or orientations. Sisters Dunite in the Middle Nooksack basin Under the influence of varying degrees of and several small intrusions in the Green weathering, soil creep, mass movement, and and South Toutle basins form the intrusive erosion by tributary streams, these rocks landforms. have been shaped into what we think of as In contrast to the nonbedded landform "normal" hill slopes. These may be of al­ group, terrains formed on strongly bedded most any slope gradient, concave, convex, rocks are distinctive because of the control straight, or a combination of shapes; may the bedding-plane directions exert on be gently or strongly dissected; and may erosion processes, especially mass wasting. possess a thick mantle of regolith or no This is most apparent in formations of regolith at all. In any case, these types interbedded stronger and weaker rocks, such of rocks usually behave more or less in a as the sandstones and shales of the Middle uniform way. Nooksack basin and the volcanics and volcani­ There are two zones of slope instabi­ clastics of the Green and South Toutle River lity in most of these nonbedded rock terrain. basins. One is along the lower slopes, where erosion Most kinds of mass movement involve Bedrock Terrain 11

sliding along a failure surface. Bedding Where dense, hard rock (such as ande­ and joint planes are pre-existing failure site or basalt from lava flows) is inter-. surfaces which will be utilized if stresses bedded with softer, weaker material (such tending to produce failure correspond to as clayey volcaniclastic tuffs or breccias), the directions of the discontinuities. In the weaker layers may not be able to support other words, if there are already planes of the heavier layers when lateral support is discontinuity within the mass that are removed even at low gradients. This seems approximately parallel to the direction of to be the situation between the South and shear, the mass will be more likely to fail. East Forks of the Grays River, where Goble In addition, discontinuous bedding Volcanics overlie soft Cowlitz Formation planes in competent rock, or permeable sediments, and slumping has occurred along interbeds, provide pathways for the move­ a 5 km (3 mi) zone. ment of ground water. If water pressure An important consideration in terrains builds up on the cracks, it can aid in the involving bedded rocks is the relationship failure of the overlying mass by reducing between the attitude (strike and dip) of shear stress which tends to hold the mass the bedding planes and orientation of the in place. If the water exits the rock in hill slope at a particular site. Figure 8 the form of springs or seeps into the soil, shows the possible relationships. over saturation of the soil mass may cause In figure 8A, the beds are horizontal shallow slumping or debris avalanches. or nearly so. Failure is unlikely to occur

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Figure 8.-Dia.grammatic sketches of bedrock dip and hill slope relationships. Stability dependent also on bedrock joint pattern. Joint attitude may act similar to bedding attitude. (A) Bedrock dip horizontal, slope failure unlikely; (B) Slope gradient steeper than bedrock dip, very unstable; (C) Slope parallel to dip, unstable; (D) Dip into hill, usually stable but rockfalls are more likely; and CE) Dip steeper than slope, usually stable.· 12 Forest Slope Stability Project along bedding planes, though it may still the bedding planes are called dip slopes or take advantage of joints, or create new back slopes. The steeper slopes cutting failure surfaces. In figure 88, the slope across the bedding are called scarp slopes, gradient is steeper than the dip, and the cuesta scarps, or front slopes (figure 9). bedding planes daylight out of the hillside. Dip slope and scarp slope landform This is the most unstable condition for the units have been mapped in four of the study beds, since they have no lateral support. basins. In general, dip slopes are recog­ In figure 8C, the dip and slope are nearly nized where the slope orientation is approx­ parallel (forming a dip slope): the outer­ imately the same as the bedding attitude. most bed is unstable, especially if it is Dip slopes are prone to rockslide and slump­ undercut; the lower layers retain lateral earthflow involving beds or sets of beds, support, but are nonetheless being stressed and to debris avalanches involving regolith in the direction of the slope. Extensive that may be only loosely connected to the jointing on the opposite sides of the hill underlying bedrock. (the scarp slopes) would create the same Scarp slopes, strictly speaking, are kinds of opportunities for failure (figure the slopes on the opposite side of a ridge 80). Figure 8E shows a dip steeper than the or hill from the dip slopes. In this study, slopes. Though the stress direction is still scarp slope landform units generally unfavorable, lateral support is maximized. include all slopes for which the slope The joint planes, however, may daylight out aspect is more than about 45° away from of the slope. the dip direction. On these hillsides the Erosion processes act on bedded rocks beds dip predominantly into the hill slope, to create several distinctive landforms. so lateral support is provided. The Hill slopes tend to evolve parallel with stronger beds commonly stand out on the bedding attitude, since it is usually scarp slopes as knobby outcrops or steep easier to peel off layers than to cut across free faces or bluffs. On scarp slopes, them. The result is commonly an asymmetrical the joints become more important controls ridge called a cuesta, homoclinal ridge, or on mass movement, and debris avalanche and hogback. The larger surfaces parallel to rockfall (especially on the free faces) are

Figure 9.-Diagrammatic sketch of dip slope and scarp slope. Landslide Terrain the most common forms of slope failure. An exception to the general definition by streams. and ntassf~i'. of scarp-slope units above was made in the has formed steep, unstable ~!if'Y, Middle Nooksack basin. In the Sandstone/ . walls. Incised valley landforms. Shale terrain of that basin, the scarp slopes in all of the study basins, and are somb;o, are fairly smooth hillsides, with few free the most unstable. zones in each drainage. faces. However, on slopes that are parallel It must be remembered that no classi­ to the strike of the strata, the strong sand­ fication scheme is perfect, nor are the maps stone beds form prominent ribs that trend that accompany this report substitutes for downward and across the slopes. These site-specific studies for individual timber landforms are defined as ribbed slopes in sales, roadways, etc. Boundaries between the Middle Nooksack basin. map units are commonly approximate. In Because strata have been folded and not addition, certain factors important to slope just tilted, simple cuestas are uncommon; stability do not appear on the maps, but in the study basins, they are limited to should be observed and measured in site the eastern half of the Grays River basin and studies. For example, slope gradient is perhaps the South Toutle. Bedded rocks in an extremely critical yet locally variable the Middle Nooksack, Green and South Toutle factor. Profile concavities (hollows) con­ basins have been folded so the attitude of centrate water, and thus are preferred the bedding changes from place to place. locations for failure, yet they were too Furthermore, erosion of these folds has small to be mapped as separate landforms at created a variety of slope-dip relations. the 1:24,000 scale used. Geologic structure Therefore, it may be necessary to determine is important, but this information must be the relationship of the bedding to the slope obtained from geologic maps or in the field. at a particular location in order to assess Whether or not the debris from a slide the site specific potential for bedding­ reaches a channel will control its impact related mass movement. Attitudes and fold on stream quality, as will the size and geometry are given on most geologic maps, or breakdown of the debris particles if it does. may be measured in the field. Rib and Liang (1978, p. 37) noted that "landslides can occur on almost any landform LANDSLIDE TERRAIN if the conditions are right." They summar­ ized the areas susceptible to failure as: The third terrain type is composed of 1. Steep slopes. landforms shaped by presently or recently 2. Cliffs and banks undercut by streams (Holocene) active mass movement processes. or waves. These are generally called landslide terrains, 3. Areas of drainage concentration and in the broad sense of the term. This group seepage. includes prominent slump topography, and 4. Areas of hummocky ground. older, less distinct flow topography having 5. Areas of concentration of bedding hummocky ground lacking some aspect of slump planes and fractures. landforms. It also includes incised valley 6. Recent landslides. 14 Forest Slope Stability Project

NORTH CASCADES~MIDDLE FORK OF THE in/hr) in this region (Miller and others, NOOKSACK RIVER AREA 1973). The Nooksack River basin is located in The north Cascades are made of old the northwestern corner of the north Cascades, metamorphic rocks (Paleozoic Skagit suite in l~hatcom County (figure 10). The Middle gneiss; Shuksan suite phyllite, greenschist Fork of the Nooksack River begins at the and blueschist), old marine conglomerates, Deming Glacier on Mount Baker, and flows sandstones, shales, and submarine volcanics generally northwestward to its confluence (Paleozoic Chilliwack Group; Mesozoic with the North and South Forks near the edge Nooksack Group}, younger continental sand­ of the Puget Lowland. The topography in­ stones and shales (Tertiary Chuckanut and cludes craggy alpine slopes, subalpine Swauk Formations}, and volcanics and meadows, deep canyons, U-shaped glacial plutonic instrusions of a variety of ages valleys, rounded foothills, and irregular and compositions. These rock types have glacial surfaces. been telescoped together by a series of Phyllites and schists (Shuksan suite), thrust faults, and are cut by many normal submarine volcanic and sedimentary rocks faults (Misch, 1966, 1977). Mount Baker and (Nooksack Group, Chi11iwack Group}, and pods Glacier Peak have erupted repeatedly within of altered mafic and ultramafic rocks crop the last half million years, and their vol­ out in the eastern and southwestern parts of caniclastic debris has been deposited in the basin. These rocks have been metamor­ the valleys of the Nooksack, Skagit, phosed, folded and broken by low-angle Suiattle, Sauk, and Stillaguamish Rivers. faulting. The effects of lithology and The north Cascades were lifted to high structure on landforms are extremely variable elevations in the last few million years, on these rocks. Bedded sandstone and shale and as a result rivers and glaciers have cut (Chuckanut Formation) crop out in the north­ deeply into the range, producing high relief western part of the basin (Vonheeder, 1975). and steep slopes susceptible to mass failure These rocks have been compressed into broad, (see, for example, Piteau, 1977). The area northwest-plunging folds; as a result, the north of Skykomish River was covered by con­ dip of the rocks changes, and this strongly tinental ice, while most of the southern influences local landforms and mass movement part of the region was covered by alpine and characteristics. The Twin Sisters massif valley glaciers. Till is ubiquitous, and is made of dunite (crystalline olivine) that outwash and glacier-margin deposits are intruded the crust during the Tertiary and common along the mountain front and in the now stands at high elevation. Andesite flows river valleys. Glaciation and intense erosion from Mount Baker, no more than 400,000 years have left only thin soils on most slopes. old, form ridgetops and the cone itself in Average annual precipitation in the the east. Mount Baker has also been the north Cascades ranges from 180 cm to about source of large mudflows that have flowed 300 cm (70 to 120 in) on the higher moun­ down the Middle Fork twice.within the last tains, with maxima of 380 cm (150 in) on 6,000 years (Hyde and Crandell, 1978). Mount Baker and 400 cm (160 in) near Stevens Glacial activity has been widespread Pass. The intensity of the 2-year, 24-hour throughout the Nooksack basin. Fluvial and storm is generally 7.5 to 14 cm/hr (3 to 5.5 glacial erosion has cut deeply into the rocks North Cascades-Middle Fork of the Nooksack River Area 15

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Figure 10.-Map of Middle Fork of the Nooksack River study area. of the basin. The highlands and valleys were steep scarps. The river bottom contains occupied by alpine glaciers during the reworked mudflow deposits and slump debris; a Pleistocene, and small glaciers are still broad floodplain begins where the river leaves active on Mount Baker and Twin Sisters. The its canyon. entire basin was covered several times by the Most of the nonfederal land in the Middle Cordilleran ice sheet. The glaciers left Nooksack basin has been logged, much of it discontinuous layers of till on the slopes within the past two decades. Most of the affecting subsurface drainage. The contin­ remaining old growth is located on Forest ental ice also left till and outwash in the Service land in the eastern half of the basin. lowlands (Mosquito Lake and Kulshan areas), Mapping for this report does not include the and on top of the Van Zandt Dike. Thin soil Forest Service land. development on these deposits and disrupted The Middle Nooksack basin is divided subsurface drainage results in poor slope into four terrain groups. The first is made stability in many areas. up of landforms shaped by glacial and fluvial In the Middle Fork valley, deposits of process; two are terrains developed on glacial till and outwash, volcanic mudflow different kinds of rocks and structures. debris, and sediments deposited in landslide­ The fourth group is composed of large land­ dammed lakes have been incised by the river, slide deposits and active stream incision leaving ~rominent terraces and benches with landscapes. 16 Forest Slope Stability Project

ALLUVIAL~GLACIAL TERRAIN and have minimal potential for mass movement. Valley benches (B): Fragments of former Floodplains (F)*: This unit includes the fill terraces and bedrock benches remain in active floodplain along the lower reach of the many of the valleys, especially the middle and Middle Nooksack. Coarse sediments (mostly upper reaches of the Nooksack, along Clearwater sand and gravel) are being deposited and re­ Creek (figure 11), and in the northeastern worked by the braided channels. Since this corner of the basin. These benches are made area is nearly flat, there is no slope of glacial till and outwash, alluvium, and instability, and the only potential mass mudflow deposits, and may be mantled with movement hazard is from slides or debris colluvium. Most of the benches are flat or torrents entering this zone from adjacent nearly so, but in some places they may slope highlands. as much as 25° (e.g. lateral moraines along Terraces (T): This landform includes the upper Nooksack). They are mostly stable, low terraces along the lower reach of the except where incised by stream action. Nooksack (where a low scarp, about a meter Alluvial fans and cones (Af): Streams high, separates it from the valley bottom), debouching from glacially oversteepened slopes and several small terraces and terrace frag­ onto flatter valleys have formed fans and ments in the upper Nooksack and middle Clear­ cones along the Middle Nooksack, Clearwater water Creek. The material is predominantly Creek, and in the Mosquito Lake lowlands. The alluvial, but includes some mudflow deposits, fans are made of unconsolidated, mostly coarse especially in the upper Nooksack where a vol­ alluvium, talus, and debris torrent deposits canic mudflow occurred about 300 to 500 years (boulders, gravel, sand, soil), and are ago (Hyde and Crandell, 1978). The terraces moderately steep (15 to 35°t). The streams are nearly flat, subject to occasional flooding, that formed them are generally now incised

Figure 11.-Stereo pair of the Clearwater Creek area. Note the valley benches of glacial sediment along the bottom of the valley, also the large landslide on the slope of Big Slump Mountain (in bedded sandstone-shale terrain). * Symbols used on landform maps. t Slope gradients cited are generalized-some slopes within a given landform unit are outside the ranges quoted here. North Cascades-Middle Fork of the Nooksack River Area 17

into the fans, which suggests that a few large outwash, and alluvium. These are mainly landslides and/or debris torrents are re­ depositional zones, and are stable exce~t sponsible for most of the material in them. when incised. The fans and cones were deposited near Cirque floors (Glf): Cirque glaciers the angle of repose, and so are fairly stable occupied higher ridges of the basin during where undisturbed. However, they are subject glacial ages, and still exist in the Twin to mass failure (avalanche or slump) when Sisters and on Mount Baker. Cirque floors undercut by streams or road construction. are flat to gently sloping, and usually cut Continental glacial deposits (Glc): into bedrock though some may contain talus, Continental ice sheets overrode all of the till, and thin soils. These areas are stable, Middle Nooksack basin, and in the western but subject to rockfall and avalanche from the part of the area left distinctive deposits steep slopes surrounding them. and landforms behind. The deposits include Cirque headwalls (Glh): These landforms compact, poorly drained, sandy to bouldery are steep to nearly vertical walls cut back till, and loose, well-drained recessional into ridges by cirque glaciers. The bedrock outwash of sand and gravel. The thickness of walls are mostly unvegetated, hold little the material is variable, and bedrock crops soil, and thus are exposed to intense weather­ out in places. The top of the Van Zandt Dike ing. Debris avalanche and rockfall are is a rolling surface of glacial till. Outwash common on these slopes. forms a hummocky surface in the Mosquito Lake lowlands and on the lower hill slopes east of MATURE BEDROCK TERRAIN Kulshan. Flat to rolling till-covered highland areas above Canyon Creek and in This group includes most of the area Dailey Prairie have also been included in this underlain by pre-Cenozoic rocks. The unit. lithologies include phyllite, serpentinite, The low to moderate slopes (most less gabbro, blueschist, slate, metasandstone, and than 35°} of this unit do not allow much mass dunite; most are foliated, folded, and faulted. movement, despite the irregular drainage and The landforms developed on these materials are low strength of these materials. There is locally variable (partly due to glacial some slump-earthflow in the northwestern part modification), and range from moderate of the basin; the boundary between glacial footslopes to alpine crags (figure 12). The and slump terrain east of Kulshan is indistinct. breakdown of the rocks and soil development Most mass erosion is due to incision of the depend on rock type; the phyllite, blueschist, unconsolidated materials by streams. slate, and metasandstone weather into boulders Glaciated valleys (Glv): The upper parts and gravel, while the dunite, serpentinite, of the va 11 eys of Canyon Creek, Porter Creek. and gabbro break down into granular particles Clearwater Creek, and Warm Creek retain and clays. distinctive U-shapes inherited from scour by Moderate slopes (Mm): This unit com­ valley glaciers. Several valleys on the prises most of the area of mature bedrock north and west side of the Twin Sisters massif terrain; it is formed on all of the rock still have active glaciers at their heads. types named above except dunite. Slopes are The bottoms of these valleys are relatively moderate (15 to 40°) and usually stable, and flat, and may contain thin deposits of till, the topography is rolling in some places· 18 Forest Slope Stab ii ity Project

till; slopes are moderate (5 to 35°). This area and the associated alpine zone are mapped as separate landforms because dunite weathers into a fine, yellow silt and black sand that are easily eroded from streambanks, roadbeds, and roadcuts by rainsplash and surface water, and cause major sedimentation and turbidity problems in streams. Mass wasting is a minor source of sediment in this unit. Twin Sisters alpine zone (Mds): The Figure 12.-Mature bedrock terrain with steep craggy slopes of the Twin Sisters make moderate slopes (Mm) and craggy, talus-covered steep slopes (Ms) on up this unit (figure 13). Because soils Grouse Ridge. formed from dunite are unfavorable for plant growth, timberline is lower on the Twin (flow topography?), such as Bowman Mountain. Sisters than it is on nearby mountains. There are a few large slumps, commonly where Active glaciation, stream erosion, rockfall, slopes have been oversteepened by glacial and avalanching all contribute to the rapid erosion. Debris avalanching is a minor prob­ mass erosion of these highly unstable slopes. lem. Movement potential is partially con­ trolled by the dip of bedding and/or foliation, which are locally variable and should be examined in the field. Steep slopes (Ms): Steeper bedrock slopes (Marmot Ridge), rocky knobs (Bowman Mountain), undercut slopes (along the Middle Nooksack), and craggy mountains (Grouse Butte, Groat Mountain, and Grouse Ridge) have been included in this unit. Some of these are the headscarps of large slumps, and some are large Mount Baker lava flows. Most slopes Figure 13.-Mature bedrock terrain of the are nearly bare of soil and vegetation, and Twin Sisters Mountain alpine (Mds) and foothills (Md) landforms. Daily are thus exposed to intense weathering. The Prairie and continental glacial (Glc) steep (35 to 80°) slopes fail mostly by rock­ landform in middle distance. View from Bowman Mountain. fall and debris avalanche. There are also some snow avalanche chutes, and talus fields are corrmon. SANDSTONE-SHALE TERRAIN Twin Sisters foothills (Md): The Twin Sisters massif is composed of dunite, an Most of the northwestern part of the intrusive igneous rock made of granular study basin is underlain by interbedded olivine crystals. This landform unit includes well-cemented sandstone and shale of the the lower slopes of the Twin Sisters. The Chuckanut Formation of Paleocene age. These materials here are dunite and dunite-rich rocks may have zones of weakness between North Cascades-Middle Fork of the Nooksack River Area 19 adjacent beds, or thin, weak shale layers between thicker, stronger sandstone beds. Joints, usually perpendicular to the bedding, can also act as failure planes. The weather­ ing characteristics are an important factor regarding stream sediment problems. The breakdown of these rocks occurs in two stages. The sandstones tend to break into large blocks and boulders, so debris avalanches and torrents commonly result in piles of coarse, boulder­ and cobble-size material. Eventually the sandstone blocks break down into small flakes Figure 111.-Dip slopes in sandstone-shale and individual sand grains, 1>1hich are then terrain ( north of Canyon Lake) . Hummocky topography and slump­ available for stream transport and sedimenta­ earthflows visible in the logged area. tion. The shale layers break down into small Note edges of sandstone beds crop­ ping out where slopes are steeper chips, and then weather into silt and clay than the dip of the bedrock. and are easily transported into streams. The regolith formed on the Sandstone~Shale terrain The steeper slopes at the headwaters of is mostly sand or silt, fairly cohesionless, Porter Creek and east of Clearwater Creek and easily eroded by either fluvial or mass have more debris avalanches and torrents, wasting processes. The rocks making up this due to small masses of soil or regolith terrain have been warped into open, northwest­ becoming saturated at the till or bedrock plunging folds, so the dips of beds vary interface and losing sheer strength. Smaller greatly within the study area. Since the areas of dip slopes seem fairly stable, direction of dip, relative to the slope, particularly where the slope and dip are controls the landform and mass-wasting sha 11 ow. processes dominant in a given area, three of Since the exact relationship between the units in this terrain are defined on the dip angle and slope gradient is critical to basis of bedding attitude. slope stability in this landform, it is Dip slopes (Sd): A high potential for important to determine these parameters from mass movement exists where the dip of bedding geologic maps or, preferably, in the field. planes is in the same direction as the slope Beds with dips much steeper or shallower (figures 8, 9, and 14). This unit has the than the slope will be more stable than greatest potential for rockslide because those beds that are nearly parallel to the large slabs of bedrock may become detached slope. Undercutting by streams and road and slide downslope as planar failures. construction can destabilize large hill Larger slump-earthflows also occur in this slope areas. landform, again from the dip slope relation­ Scarp slopes (Sa): Slopes that cut ship. The exact mode of failure in a speci­ across bedding planes are not as liable to fic location depends largely on the slope planar slides on the dip slopes. However, gradient. The lower gradients north of since the sandstone and shale are jointed, Canyon Creek produce mostly large slump they may fail by rockfall along fractures that earthflows, with a few large rockslides. cut across the bedding. In addition, thin 20 Forest Slope Stability Project cohesionless regolith is subject to debris poorly vegetated slopes are failure, and many debris avalanches and avalanching and rockfall. torrents occur on this unit, especially in the headwalls of small streams (such as the west side of Clearwater Valley), and in logged and roaded areas (Porter Creek Valley). Slope gradient is a critical factor in stability, along with dip and joint angles; slopes above 20° should be considered potentially unstable. In general, though, this unit has a very high potential for shallow mass movement. Ribbed slopes (Sx): Intermediate between the two preceding slope types are slopes in which the outcrop pattern of the beds Figure 16.-Large slump-earthflows {E) in the Middle Fork of the Nooksack River trends down and across the hill slopes; basin. Older slump-earthflow on commonly, the resistant beds form ribs that left younger on right. protrude from the hill slopes, such as in the slope south of Porter Creek (figure 15). LANDSLIDE TERRAIN Slopes in this unit are subject to debris failure and rockfall, but to a lesser degree The fourth terrain group is composed than the scarp slopes. of landforms shaped predominantly by mass erosion processes. These include the deposits of large landslides, and zones of undercutting and mass wasting along major streams. Slump landforms (Ls): Large landslides occur on most of the terrains in the basin. Some are probably early postglacial in age (about 10,000 years old), while some, such as the slide on Big Slump Mountain, appear fairly young (hundreds of years old?). The slump landforms are recognized by broken, jumbled rock and regolith. Some of these Figure 15.-Resistant edges of sandstone beds landslides are stable; others, such as the forming ribbed slopes (Canyon Creek valley area). large slide on Big Slump Mountain may have been occasionally reactivated by stream Undercut slopes (Su): In several undercutting. Some are active, such as the locations undercutting of slopes, rather than slide on the south side of Grouse Butte, on bedrock attitude, controls slope form. Under­ which there are tension cracks over a dis­ cutting can be caused by stream action, as tance of 125 m (410 ft). In any case, the along the Middle Nooksack and Porter Creek, or materials have been weakened by past move­ by mass movement, as above the two large land­ ment, and the potential exists for failure slides on Big Slump Mountain (figures 3 and if they are undercut by natural processes or 16). These steep (35 to 55°), commonly human activity. Nort~ Cascades-Middle Fork of the Nooksack River Area 21

Incised valleys (Li): Most of the the middle reach of the Middle Nooksack. Since larger streams in the Middle Nooksack basin the end of the ice age, the river has been are actively downcutting in some reaches. cutting down through glacial and mudflow As a result, the streams are oversteepening deposits, some bedrock, and perhaps through slopes by undercutting them, thus destabi­ landslides that have dammed the river during lizing large areas of lower hillsides. postglacial time. The canyon is up to about These areas of active and recently active 150 m (500 ft) deep, and its sides show the undercutting are mapped as incision zones. steep (30° to 80°) scalloped forms indicative The largest single incision zone is along of slumping. Areas along the lower reach of

TABLE l.~Generalized instability of Middle Fork of the Nooksack River landforms

Landform Low Moderate Alluvial-glacial terrain Floodplains (F) X Terraces (T) X Valley benches (B) X Alluvial fans and cones (Af) X Continental glacial deposits (Glc) X Glaciated valleys (Glv) X Cirque floors (Glf) X Cirque headwalls (Glh) X

Mature bedrock terrain Moderate slopes (Mm) X Steep slopes (Ms) X Twin Sisters foothills (Md) X Twin Sisters alpine zone (Mds) X

Sandstone-shale terrain Dip slopes (Sd): O - 20° X 20 - 30° X 30°+ X Scarp slopes (Sa): O - 20° X 20 - 30u X 30°+ X Ribbed slopes (Sx): O - 30° X 30°+ X Undercut slopes (Su) X

Landslide terrain Slump landforms (Ls) X (X) Incised vall~ys (Li) X

(X) Indicates active landslides, or unfavorable slope or hydrologic conditions. 22

landfoT'm8

C ~ Flood

X X X X X X Continental glacial deposits (G1c) X Glaciated valleys (Glv) X X Cirque floors (Glf) X X Cirque headwalls (Glh) X X X X

Mature bedrock terrain Moderate slopes (Mm) X X Steep slopes (Ms) X X X X Twin Sisters foothills (Md) X Twin Sisters alpine (Mds) X X X

Sandstone-shale terrain Dip slopes (Sd): 15 - 25° X X 25°+ X X X X Scarp slopes (Sa): O - 20° X 20°+ X X X Ribbed slopes (Sx): 10 - 30° X 30°+ X X X Undercut slopes (Su) X X X

Landslide terrain Slump landforms (Ls) X X Incised valleys (Li) X X X X X X

E - Slump-earthflow Rf - Rock fa 11 A - Debris avalanche C - Snow avalanche D - Debris torrent Dep- Deposition by mass wasting from St- Stream erosion adjacent hill slopes R - Rockslide Flood - Stream flooding the Nooksack are also mapped as incision zones stream quality degradation due to mass move­ where the river is impinging against and ment. They are subject to undercuttinq, slump, undercutting hill slopes (sidecutting). Most and debris avalanche, and the debris is of the tributary streams are incising their deposited directly into the streams. It middle reaches. In Dailey Prairie, the high­ should also be noted that the limits of this lands in the eastern part of the basin, and unit, as mapped, do not include areas on some other areas, glacial deposits and benches terraces, benches, or hill slopes above the are being incised by small streams. "Li" zones, which may be destabilized by Incision zones have great potential for further incision-related failure. Central Cascades-Green River Area 23

GENERAL INSTABILITY AND HAZARDS rocks in the north Cascades, mostly in the hills near the confluence of the Skykomish Table l summarizes the general instabil­ and Snoqualmie Rivers and near Barlow Pass. ity ratings and Table 2 lists the hazards These rocks will behave like those in the associated with the landforms for the Middle Green River and South Toutle basins. Nooksack. From a management standpoint, the most crucial units are: CENTRAL CASCADES-GREEN RIVER AREA (l) All of the bedded Sandstone-Shale terrain (Sd, Sa, Sx, Su), in which the South of the Skykomish River, the potential for debris avalanche and torrent, Cascades are composed predominantly of rocks rockslide, and slump-earthflow is ~igh, and younger and less deformed than those making where much of the harvestable timber is up most of the north Cascades. This area located. has been uplifted less, and therefore in­ (2) Incised valleys (Li), in which mass cised less. The result is a region of erosion is most active, and has the most generally less extreme relief than the immediate impact on the large streams of the north Cascades. basin. The rocks of the central Cascades in­ (3) The cirque headwalls (Glh), steep clude Mesozoic marine sedimentary and vol­ bedrock slooes (Ms), and Twin Sisters alpine canic rocks (Nooksack Group) and early and zone (Rds), which have steep bare slopes middle Tertiary marine and continental subject to avalanche and rockfall, but have sediments (Puget Group, McIntosh and Skookum­ little timber. chuck Formations). However, most of this In the western north Cascades only a region is made up of volcanic and volcani­ couple of important rock types occur that clastic rocks, spanning the Tertiary and are not represented in the Middle Nooksack Quaternary in age (Mount Catherine tuff, basin. Crystalline rocks, such as granitics Keechelus volcanics, and Cougar Mountain and gneisses, form large masses near the formation in the Snoqualmie-Cedar-Green crest of the range east of Mount Baker Rivers area; Ohanopecosh, Stevens Ridge and (Skagit Gneiss of Misch and Chilliwack Fifes Peak Formations and recent andesites batholith), south of Darrington (Squire in the Mount Rainier area; Northcraft Creek stock), and near Stevens and Snoqualmie Formation in the western foothills). The Passes (Index, Grotto, and Snoqualmie batho­ rocks have been intruded by plutons of liths). Since they are granular and unbedded, various sizes (Snoqualmie batholith, Tatoosh these rocks behave in ways somewhat similar pluton, etc.). This area underwent folding to the Twin Sisters rocks (except that the and uplift in the middle Tertiary and uplift minerals of granitics weather slowly to· in the late Tertiary and Quaternary, though sand instead of silt and clay, as the olivine to a lesser degree than the north Cascades. in dunite does) and the intrusive landforms Folds are broad and open, and faults have in the Green River and South Toutle basins. relatively small offsets. See Megahan and Kidd (1972) and Durgin (1977) The central Cascades were not as for discussions of erosion on granitic rocks. intensely glaciated as the north Cascades. There are also some Tertiary volcanic Continental ice piled up against the 24 Forest Slope Stability Project mountains in the Puget Lowland, and large most of the region, but may be as high as alpine icecaps covered the range from 15 cm/hr (6 in/hr) at the higher elevations. Mount Rainier north, extending into the The Green River basin lies almost valleys of the Snoqualmie, Cedar, Green, exactly in the center of the central Cascades, White, Puyallup, Nisqually, and Cowlitz in southeastern King County. The river Rivers. However, the high ridges were never begins on the Cascade crest (at one of its overtopped. The less intensive glaciation lowest points) and flows northwest to the has allowed longer weathering and the mountain front, then west across the Puget development of somewhat deeper soils. Lowland drift plain, and north to Puget However, stream erosion, aided by glacial Sound. The study area (figure 17) for this scour in the upper valleys, has created high project is upstream of the Howard Hansen relief and steep slopes throughout this reservoir. The basin contains low- and high­ region. The interbedding of flow rock and relief topography, and some craggy subalpine volcaniclastics provides many zones of weak­ terrain. ness, which commonly become the failure sur­ The rocks within the study area are faces of large landslides throughout the . mostly volcanics; they include interbedded western Cascades (Swanson and Swanston, 1977). flows, breccias, and tuffs of andesite to Annual precipitation in this province rhyolite (the Huckleberry Mountain formation, ranges from about 130 to 200 cm (50 to 80 in), Eagle Gorge andesite, Stampede tuff, Snow with a maximum of 360 cm (140 in) on Mount Creek formation, and Cougar Mountain for­ Rainier. The intensity of the 2-year, 24-hr mation of Hammond, 1963). The rocks are storm is 7.5 to 13 cm/hr (3 to 5 in/hr) in compressed into gentle folds, trending north-

R9E R /0 E R II E Rl2E

T21N PASS

T20N

GREEN

SCALE T/9 N MILES 0

0 4

KILOMETERS

Figure 17.-Map of Green River study area. Central Cascades-Green River Area 25 west to west-northwest. These folds control area and in the south-central part of the the dip of the rock, and thus the location basin. of certain landforms (such as dip slopes The Green River basin has been divided and plateaus) and mass movement types. into six terrain groups. The first consists In addition, there are several small intru­ of landforms created or significantly shaped sions that form the craggy topography of by fluvial and glacial processes. Three Kelly Butte, Rooster Comb, Goat Mountain, terrains are classified on the basis of and Meadow Mountain. landscapes produced on three different The exact extent of glaciation in the combinations of the volcanic and volcani­ Green River basin is unknown, but during clastic rocks that underlie most of the glaciation, the cirque glaciers expanded basin. A fifth group consists of intrusive into the upper valleys of Sunday Creek, the igneous rocks, large free faces, and asso­ upper Green, and their tributaries, leaving ciated talus zones; and the sixth group is many cirque basins located on the northern made up of large-scale slump terrain and sides of high ridges. The continental ice valley incision landforms. sheet blocked drainage through the preglacial valley that extends northwest from Howard ALLUVIAL-GLACIAL TERRAIN Hansen reservoir, and deposited outwash in the valley mouth. A drift plain or lake Floodplains (F): Due to the local base formed in the preglacial valley and in the level created by Eagle Gorge, the bottom of Green River valley, and drainage was diverted the Green River valley is wide and flat to the west, cutting Eagle Gorge (Mackin, 1941). through most of the study basin. It is The Green River flows mostly on alluvium from especially wide at the confluence of the its confluence with Sunday Creek; valley in­ Green River and Sunday Creek, where glacial cision is occurring on tributary streams. erosion and subsequent fluvial sedimentation Since most of the study basin has not may have combined to create a large valley been subjected to glaciation, weathering is bottom. The materials in this unit are fairly well advanced, and soil depths are young fluvial sediments. Slopes are nearly controlled by fluvial and mass erosion rates. flat, so there is no stability problem in A recent geomorphic event of note was the this zone. storm of December 1977. Heavy rain and rapid Tributary valleys (V): Narrow, flat snowmelt caused a large number of debris valley bottoms have been formed in some avalanches and torrents, flooding on most reaches of Smay Creek, Sunday Creek, Snow major streams, and many road and bridge wash­ Creek, and the upper Green River, with the outs. In 1981, the effects of this event deposition of small amounts of alluvium. are still visible in the form of stream These valleys are subject to mass movement diversions, piles of debris and sediment, from adjacent hillsides. and large channel bars. Terraces (T): Areas of flat to gently Most of the study basin is in a checker­ sloping topography along the Green River board of private and federal ownership; have been included in this unit. The private owners have cut more of their timber surfaces are made up of old fluvial material~ than the Forest Service has. Most of the plus glacial deposits and bedrock shaped by remaining old growth is in the Green Canyon fluvial processes. The terraces themselves 26 Forest Slope Stability Project are stable, but may be affected by debris walls, but are themselves stable. torrents in tributary streams that cross or Headwalls (Glh): This unit includes the debouch onto them. headwalls of glacial cirques on the ridges Benches (B): Relatively low-gradient surrounding the study basin, and also the slopes up to about 300 m (1000 ft) above slopes below the ridge of Huckleberry river level were included in this unit. The Mountain. These slopes are steep (commonly benches may be old fluvial terraces, deposits 30 to 55°, some nearly vertical), and subject of glacial outwash or glacial lake sediment to considerable debris avalanche, rockfall, plastered on hillsides, or landslide benches. and minor slump-earthflow. They are flat to gently sloping (less than 25°) and show little evidence of mass move­ VOLCANICLASTIC TERRAIN ment, though the loose materials could be destabilized if undercut. The largest landform group in the Green Alluvial fans (Af): Many fans and cones River basin is formed on volcaniclastic of unconsolidated debris have formed where deposits and interbedded lava flows (Enumclaw small, high-gradient tributaries join the formation, Huckleberry Mountain formation, Green River valley. These fans commonly Cougar Mountain formation) in which the merge with terrace or floodplain landforms volcaniclastics (breccias, tuffs, volcanic at their distal ends, so precise delineation sediments) are volumetrically greater. is lacking on some of them. Most of these Therefore, most of the slope forms are deter­ fans were deposited at gentle gradients mined by the properties of the structurally (10° to 25°), and are stable; however, they weaker volcaniclastics, though the harder, are susceptible to debris torrent deposition. stronger flow rocks do control some slope Glaciated valleys (Glv): Many high­ forms. The volcaniclastic rocks are generally elevation valleys and passes show signs of soft and crumbly, and break down quickly into glacial erosion. The upper Green River, clay, sand, or small granules. The more Tacoma Creek, Pioneer Creek, and their trib­ competent breccias break down into longer­ utaries have wide U-shaped valley floors. lasting boulders and cobbles, as do the flow The areas around Tacoma Pass, Stampede Pass, rocks. Meadow Pass and Cedar Notch are also rela­ Because the rocks of this region have tively flat, and appear to have been traversed been broadly folded, and because landforms by glaciers originating somewhere in the and their stability seem to be largely con­ crest of the Cascades. The valley floors trolled by slope-dip relations, the volcani­ are mantled with thin layers of till (no clastic terrain has been subdivided on the more than a few meters). Due to their gentle basis of structural attitude. gradients, these landforms are generally Flat bedding attitudes (Vf): In a few stable. However, they are subject to slump, locations, nearly horizontal bedding and the avalanche, and rockfall from adjacent, com­ absence of intense erosion have combined to monly oversteepened hill slopes. leave gently sloping surfaces. These small Cirque floors (Glc): Most of the cirque areas are generally stable. floors in the Green River basin are small, Dip slopes (Vd): Dip slopes in the and slope gently outward. They are subject volcaniclastic terrain are generally small in to rockfall and avalanche from adjacent head- area, since the surfaces are easily attacked Central Cascades-Green River Area 27 by stream and hill slope erosion from the debris avalanches and road failures were side. These areas are mostly gently to triggered by the December 1977 storm. moderately sloping (20° to 40°), since the dip of the bedding is almost everywhere less EAGLE GORGE TERRAIN than 45°. The slopes are straight or convex to rol 1 ing. Minor slump-earthflows of The Eagle Gorge terrain is named for weathered regolith are found on many of the the Eagle Gorge andesite, a formation of volcaniclastic dip slopes and in a few interbedded lava flows, breccias, and tuffs locations blocks of harder rock slide over in which the flow rocks predominate. The weaker layers (figure 18). Some of the Eagle Gorge andesite forms a dip slope larger slumps have been mapped as slump (approximately 15° to 25° to the north) landforms. Slump-earthflow is a major prob­ between Huckleberry Mountain and the Green lem on the slopes between Snowshoe Butte and River. Again, the structure of the rocks Sunday Creek; one slide complex is over 2.5 controls the landforms, so the terrain is km 2 (l mi 2) in area. Debris avalanche in subdivided according to the relation between this terrain is minor. slope and attitude. Dip slopes (Ed): Layers of flow rock form dip slope surfaces over much of the Eagle Gorge terrain (figure 19). The dip slopes are broadly convex, being gentle (about 5 to 15°) on the upper ridges and somewhat steeper (generally 20 to 35°) on the lower slopes. The ground surface is locally uneven, probably due to creep and earthflow of weathered pyroclastic material above the flow rocks. Slump-earthflow is the major mass movement feature in this terrain, Figure 18.-Dip slope in volcaniclastic terrain since the slope and dip are parallel. The with blocks of breccia sliding down larger landslides of this type have been toward Tacoma Creek.

Scarp slopes (Vx): This unit includes areas in which the slope cuts against or across the dip of the bedding. Most of these slopes are nearly straight, except where there is slumping or where layers of flow rock crop out. Many of the slopes in this unit are quite steep (up to 55°), and are being under­ cut by stream erosion; as a result, debris avalanche, debris torrent, and rockfall are extremely common. This is especially true in logged areas in the southeastern corner of Figure 19.-Eagle Gorge dip slopes. The flat the basin (upper Green River, Tacoma Creek, surface slopes are held up by ande­ Pioneer Creek), where a large number of site flows. 28 Forest Slope Stab ii ity Project mapped as Le; this landform is marginally stable. Scarp slopes (Ex): Where creeks have incised through the upper surface of the Eagle Gorge andesite, they have been able to attack and undercut the flows and have formed steep {25° to 55°) slopes that cut across the dip of the formation. Where flow rocks crop out on the slopes, they create steep free faces which hold up the slopes, allowing them to stand at steeper angles than volcaniclastic scarp slopes, where lava layers are not as Figure 20.-Subdued Snow Creek terrain north abundant. Because this unit is made up of of Smay Creek. Note steeper volcani­ clastic terrain (Cougar Mountain steep slopes that are being incised by tribu­ Formation) on ridge, Goat Mountain tary streams, it has a high potential for on right. debris avalanche and rockfall. There are INTRUSIVE TERRAIN also some large slump-earthflows on the Ex slopes, where large masses of rock have flowed Intrusive igneous rocks crop out in toward the streams. several places in the Green River basin. Most of these rocks probably crystallized in SNOW CREEK TERRAIN lower levels of the same volcanoes that pro­ duced the lava flows and volcaniclastic The fourth landform group is formed on deposits that make up most of the area. They tuffs, breccias, and minor flows of the Snow include altered andesite porphyry at Sawmill Creek formation. It is defined as a separate Ridge, Stampede Pass, Meadow Mountain, and unit because of the extreme mass failure Green Canyon; quartz diorite at Goat Mountain behavior exhibited by this formation. and Meadow Mountain; dacite porphyry at Green Snow Creek slopes (Sc): This unit is Canyon; and andesite porphyry at Kelly Butte, located in the north and northeast sections apd Rooster Comb (Hammond, 1963) . These roe ks of the basin, in the drainages of Smay, Friday, are crystalline, well-jointed, and unbedded. West, Snow, and East Creeks. The tuffs and Intrusive terrain (Bi}: The large intru­ breccias are altered, weathered, and weak, and sions included in this unit are characterized generally cannot stand in steep slopes. The by their steep, commonly bare slopes. They rocks form gentle to moderate (less than 30°), range in style from Green Canyon, where stream rounded slopes as a result of earthflow and erosion has exposed and maintains steep slopes hill slope erosion (figure 20), In some places, but thin soils have formed and forest veqe­ such as the northeast side of Snow Creek, large tation survives, to buttes like Meadow Mountain, areas are involved in a broken, hummocky, com­ Goat Mountain, Kelly Butte, and Rooster Comb plex slump-earthflow landscape. The Snow Creek (figure 21) that form prominent knobs on which terrain is also highly susceptible to avalanche bedrock and talus make up most of the ground failure, and many major debris avalanches and surface. Rockfall and debris avalanche are debris torrents were triggered in this unit the dominant mass wasting processes active on by the 1977 storm. these slopes. Central Cascades-Green River Area 29

the edges of the dip slope surfaces, where large masses have slid into the stream valleys. Both kinds should be considered subject to continued activity or reactivation. Incised valleys (Li): Because the Green River flows in a wide floodplain through most of the study reach, incision zones mapped in the main valley are caused by sidecutting rather than downcutting. Undercutting of the hillsides occurs where the meandering river impinges on valley walls or the edges of Figure 21.-lntrusive terrain (Rooster Comb) terraces and benches. Large incision zones with rockfalls forming talus fields have also been mapped where tributary streams, below rock outcrops. Snow Creek terrain north (right) of ridge. especially West Creek, Snow Creek, and the upper Green, are cutting into adjacent slopes. LANDSLIDE TERRAIN Smay Creek and Sunday Creek are downcutting to a lesser extent. (Many of the Eagle Gorge Mass erosion landforms of the Landslide scarp-slopes also behave like Li zones). All Terrain include large slump-earthflows and of these incision zones have high potential valley-incision zones. They are divided into for slump, debris avalanche, and rockfall. three units: Slumps on volcaniclastic rocks (Lv): GENERAL INSTABILITY AND HAZARDS These landforms are located mostly in the northwestern and southeastern corners of the Tables 3 and 4 summarize the landforms study basin. The surfaces are rolling to instability ratings and hazards in the Green hummocky, and in many cases are fairly steep River basin. The terrain units most suscep­ (up to 45°). Most of these slumps are related tible to mass movement are: to the dip of the bedrock, but some large (1) The Snow Creek terrain (Sc), in which failures cut across the structure. These there is major slump-earthflow and debris mechanically weak rocks are subject to con­ failure activity. tinued failure and reactivation, as evidenced (2) The scarp slopes on both Eagle Gorge by the number of smaller slumps and debris (Ex) and volcaniclastic (Vx) terrains, and avalanches that are located on old slump incision zones (Li), in which direct under­ terrain. cutting by streams and active mass wasting Slumps in Eagle Gorge terrain (Le): are taking place, and where debris has direct Several large slump-earthflows are located on access to the streams. the Eagle Gorge andesite. Some of these are (3) The headwalls (Glh) and intrusive on the dip slopes, and are earthflows of vol­ terrain (Bi), in which steep slope materials caniclastic material and regolith probably are subject to avalanche and rockfall. moving over a hard rock layer; these slides (4) Slump terrain (Lv, Le) and dip slopes are probably restricted to a shallow surface on volcaniclastic rocks (Vd), in which slump­ zone. Other slump-earthflows have formed on earthflow is active or potentially so. 30 Forest Slope Stability Project

TABLE 3. -1,'ene Y'a l foed ins tabi U ty of DNwn R1:ver landforms

Landform Lov1 Moderate High Alluvial-glacial terrain Floodplains ( F) X Tributary valleys (V) X Terraces (T) X Benches (B) X Alluvial fans (Af) X Glaciated valleys (Glv) X Cirque floors (Glc) X Headwalls (Glh) X

Volcaniclastic terrain Flat bedding altitudes (Vf) X Dip slopes (Vd) X (X) Scarp slopes (Vx) X

Eagle Gorge terrain Dip slopes (Ed) X Scarp slopes (Ex) X

Snow Creek terrain Snow Creek slopes (Sc) X

Intrusive terrain Intrusive terrain (Bi) X

Landslide terrain Slumps on volcaniclastic rocks (Lv) X (X) Slumps in Eagle Gorge terrain (Le) X (X) Incised valleys (Li) X

(X) Indicates active landslides, or unfavorable slope or hydrologic conditions.

Two major groups of rocks relevant to on dip slopes and, where undercut, debris the central Cascades, but not represented avalanches in regolith on steep slopes. See in the Green River basin, are nonvolcanic the discussion of the Grays River basin for sedimentary rocks and young volcanics. Sedi­ treatment of analogous rock types. mentary rocks crop out especially in the The young volcanic rocks of Mount Rainier northwestern (Nooksack Group) and western form a major landform in the central Cascades. (Puget Group, McIntosh and Skookumchuck As with Mount Baker in the Middle Nooksack Formations) margins of the province. These basin and Mount St. Helens in the South Toutle units will behave much like the bedded basin, the most important mass movement hazard volcanic and volcaniclastic rocks of the from Mount Rainier comes from the occasional Green River basin: large slump-earthflows but catastrophic debris flows that can be Southern Cascades-South Fork of the Toutle River Area 31

TABLE 4.~Types of hazards associated with Green River landforms

Landform E A D St R Rf C ~ Flood Alluvial~glacial terrain Floodplains (F} X X X Tributary valleys (V) X X X X Terraces (T) X Benches (B) X Alluvial fans (Af) X X Glaciated valleys (Glv) X Cirque floors (Glc) X X Headwalls (Glh) X X X X

Volcaniclastic terrain Flat bedding altitudes (Vf) Dip slopes (Vd) X X Scarp slopes (Vx) X X X X

Eagle Gorge terrain Dip slopes (Ed) X Scarp slopes (Ex) X X X X X

Snow Creek terrain Snow Creek slopes (Sc) X X X X X

Intrusive terrain Intrusive terrain (Bi) X X X

Landslide terrain Slumps on volcanistic rocks (Lv) X X Slumps in Eagle Gorge terrain(Le) X X Incised valleys (Li) X X X X X X X

E - Slump-earthflow Rf - Rockfall A - Debris avalanche C - Snow avalanche D - Debris torrent Dep- Deposition by mass wasting from St- Stream erosion adjacent hill slopes R - Rockslide Flood - Stream flooding

triggered by volcanic activity, landslides SOUTHERN CASCADES-SOUTH FORK OF THE onto glaciers, or glacier outbursts. Crandell TOUTLE RIVER AREA (1971) has discussed these debris flows, which The southern Washington Cascades (south have occurred in the valleys of the White, of Mount Rainier to the Columbia River) Puyallup, and Nisqually Rivers in postglacial generally consist of a layered sequence of time. Tertiary volcanic deposits. Uplift and 32 Forest Slope Stability Project deformation have been less than to the north, or Green River basins except in the glaciated resulting in relatively moderate relief headwaters and steep eroded valley sides near throughout the region except for a few high Signal Peak. peaks. The bedrock is dominated by layered The rocks of the southern Cascades area volcanic rocks typical of the southern are andesite and basalt flows and volcani­ Washington Cascades. The study area clastic rocks of the Goble Volcanics and the (figure 22) can be divided into two parts Ohanapecosh Formation. Along the Columbia from west to east based on the geology. Gorge the Eagle Creek Formation of Miocene The western half of the drainage is under­ age and Yakima Basalt are found. Throughout lain by Miocene lava flows, flow breccias, the southern Cascades small granitic plutons and tuffs (similar to the Columbia River are also found. Quaternary volcanism has Basalt mapped farther to the west), and resulted in two large stratovolcanoes Eocene basalt, andesite, and pyroclastic (Mount St. Helens and Mount Adams), numerous flows, and volcaniclastic sediments (Hatchet cinder cones, and lava flows associated with Mountain Formation and Goble Volcanics). these volcanic centers. Tephra deposits are The eastern half is underlain by the middle widespread in places near the large volcanoes Eocene to early Oligocene andesite and and in places quite deep. Recent Mount St. basalt flows and volcaniclastic rocks of Helens eruptions have again spread tephra the Ohanapecosh Formation. The very upper deposits throughout the region. reaches of the South Fork of the Toutle Moderate uplift of the southern Cascades River are covered by Mount St. Helens in L-Jashington during the Tertiary and Quater­ Quaternary volcanic tephra, pyroclastic nary resulted in broad open folds and mod­ flows, and mudflow deposits (Hammond, 1980). erate relief. Quaternary glaciers have One small intrusion of dacite forms the scoured only the major valleys. Moderately Goat Mountain plug, and a small recent lava deep soils have developed over much of the un­ flow is found on the flank of the volcano. glaciated areas, deep soils over the highly The upper reaches of the drainage have weathered volcaniclastic rocks, and succes­ undergone glaciation, and there are many cirque sive deposits of ash and pumice have buried basins. Extent of glaciation is not known but some soils close to the volcanic centers. ice probably reached well beyond Herrington Annual precipitation ranges from about Creek. The upper Toutle River has also been 114 cm (45 in) in Longview to about 250 cm subjected to lahars and pyroclastic flows, (100 in) in the Cascades. Rainfall inten­ which have traveled down the mountain flank sities for a 2-year/24-hour storm range from into the main stem of the South Fork of the 6 cm/hr (2.4 in/hr) at lower elevations to Toutle River, Sheep Canyon, Coldspring and over 14 cm/hr (5.5 in/hr) at the highest other creeks. elevations. The South Fork of the Toutle River again The South Fork of the Toutle River lies experienced deposition during renewed volcanic in western Skamania and eastern Cowlitz activity in 1980 when Mount St. Helens erupted Counties. It flows west from Mount St. Helens, after a 123-year lull in activity. A large joins the North Fork of the Toutle River near mudflow traveled down the entire length of Toutle, and flows into the Cowlitz River. the South Fork of the Toutle River valley, Relief is moderate compared to the Nooksack depositing up to 15 m (50 ft) of sand, boulders, Southern Cascades-South Fork of the Toutle River Area 33

RIE R2E R3 E R4E R5E

SOUTH FORK TOUTLE RIVER T/ON

+ + 1-

T9N

TS N

SCALE MILES 0 + 0 4 8 + KILOMETERS

Figure 22.-Map of South Fork of the Toutle River study area. and debris on the valley floor. Also smaller deposits near the volcano. A few old slump­ ash and mudflows came down the west flank of earthflows were identified on the south side the mountain near Goat Marsh. With the May of the drainage where interbeds of volcani­ 18th eruption of 1980, approximately 26 km 2 clastics are highly weathered and dip parallel (10 mi 2) of forest were destroyed and up to to the slope. These slump-earthflows are not 25 cm (8-10 in) of ash covered the eastern widespread, however. half of the South Fork of the Toutle River Logging began in the South Fork of the drainage. Toutle River valley in the late 1940's or The soils in this drainage are deep to early 1950 1 s, on the south side of the very shallow. In the western part weathered drainage. The north side began to be logged bedrock, particularly flow breccias, form in the area east of Signal Peak in the early deep residual soils. Mount St. Helens tephra 1960's. Logging has progressed eastward to has been added to the soils throughout the Mount St. Helens. Most of the lower part of drainage. Different age ash layers may be the drainage, especially the south side, is seen in most of the soils, but closer to now covered with second-growth timber. Mount St. Helens many of the older soils have The landforms of the South Fork of the been buried by ash and pumice and very thin Toutle River area are grouped into five soils have begun to develop. terrain groups: those produced by fluvial and Mass wasting is not as common in this glacial processes, three groups of bedrock drainage compared to our other study areas. controlled slopes, and those where active However, many debris avalanches occur on the erosion dominates, all with varying degrees steep slopes below Signal Peak and on pumice of stability. Forest Slope Stability Project

ALLUVIAL-GLACIAL TERRAIN

The landforms produced by depositional and glacial processes are the terraces and recent (1980) and older mud and pyroclastic flows that filled the alluvial valley at the South Fork of the Toutle River and parts of Sheep Canyon, and created Goat Marsh and the glacial valleys and cirques near the head­ waters of the river. Mudflow-alluvial valley (MF): As a result of the 1980 eruption, a large mudflow Figure 23.-Bouldery mudflow surface in the originating on the flank of Mount St. Helens South Toutle valley. traveled the length of the river and filled the entire floodplain with silt, sand, deposits are usually flat to gently sloping gravel, and boulders. The headwaters of the and stable except when undercut. South Fork of the Toutle River were filled Terraces and benches (T/8): Terraces and with many meters of debris, changing the benches, formed by changes in river level by valley bottom considerably from its previous glacial or volcanic activity, are found along form. Currently it is in the process of re­ parts of the South Fork of the Toutle River establishing a coherent drainage, resulting va 11 ey, and were mapped wflere they were not in erosion and deposition throughout the covered by the 1980 mudflow. The terraces are floor of the valley. Downstream the mudflow predominantly depositional features composed deposits filled the main river channel of unconsolidated sand and gravel, and the (figure 23), scouring some parts and mantling benches are erosional features cut into the the rest of the floodplain with mudflow valley sides by the river. Their flat surfaces debris. The South Fork of the Toutle River preclude stability problems, but where under­ has eroded a channel and reestablished it­ cut they may fail. self, but winter floods have caused tremendous Glaciated valleys {Glv): The ice-scoured, amounts of erosion and deposition throughout commonly till-mantled valley floors of tribu­ the drainage. taries to the South Fork of the Toutle River Because the mudflow is generally flat, (east of Herrington Creek} comprise this land­ there are no stability problems. However, form. Alpine glaciers at one time occupied where deposits are thick (especially near these drainages, scouring and shaping the the headwaters) stability has not been deter­ valleys in an easily recognizable U-shape very mined. Flooding is almost certain to occur dissimilar to the incised V-shaped stream and streambank erosion will be widespread. valley. The lower slopes and valley floor Older mudflows or py~oclastic flows (Mp): are mantled (in places) with till or glacial Pre-1980 mudflows and pyroclastic flows have debris. Stability problems are minimal where traveled down the mountain flank and filled slopes are less than about 25°, except when some valleys. Goat Maritt was formed by mud­ undercut. When cut, banks tend to hold flow damming, and Sheep Canyon cuts through vertical, but shallow surficial slides may pyroclastic flows. The surfaces of these pccur upslope with loss of basal support. Southern Cascades-South Fork of the Toutle River Area 35

Debris avalanches may occur on slopes over about 25 0 • Glacial headwalls-valley sides (Glh): This landform is easily recognized by the steep cirque headwalls and glacially scoured valley sides. In the South Fork of the Toutle River valley these are usually vertical or near vertical rock faces. Stability problems include rockfalls and debris and snow ava­ lanches. Till or talus may cover some of the lower slopes. Figure 21&. -Gentle dip slopes on south side DISSECTED VOLCANIC TERRAIN of the South Toutle valley.

Dissected volcanic terrain developed on now basically only the ridge tops reflect the the Eocene Goble Volcanics and the Miocene general downslope dip of the bedrock. Many lava flows. Because of the depositional pro­ old landslides and earthflows can be recognized cesses and rock type, many flow or interflow and apparently are prime erosional processes zones are brecciated or tuffaceous, and over on this slope. Slopes are generally stable the past millions of years have been deeply up to 15°, moderately stable between 15° and weathered, becoming clay-rich. Deformation 25°, and unstable over 25°. Slump-earthflows, of these flows has resulted in the originally debris avalanches, and streambank erosion may horizontal layers tilting generally northward. occur on these slopes. The structure of the lava flows has caused Dip slopes {Dvd): Recognizable dip erosion to form slopes that are controlled by surfaces are found in some areas of the Dvs the dip of the flows. On the south side of landform. These dip surfaces are the remains the South Fork of the Toutle River the slopes of resistant flows that have been eroded in generally mirror the gentle structural dip to other areas of the South Fork of the Toutle the north, resulting in long relatively gentle River drainage. The northward dip reflects slopes {figure 24). These slopes have been the general structure of the bedrock in this highly dissected so only the flatter ridges area. Slump-earthflows were identified on now reflect the general dip of the bedrock. some of these landforms and slope stability On the north side of the valley erosion has is dependent on steepness of dip. Slopes are generally cut through the lava flows, leaving generally stable up to 15°, moderately stable alternating short steep slopes cutting across between 15 and 25°, and unstable over 25°. the flows. Slope stability is·very dependent Slump-earthflow and debris avalanches may on dip of rock beds and especially the angle occur on these slopes. and direction of dip. Scarp slopes {Dvx): These are slopes Dissected slopes (Dvs): This landform that are formed perpendicular to structural is the dissected slopes of the Goble Volcanics dip of lava flows. They tend to form steep, and.Miocene flows on the south side of the almost stairstep features where competent South Fork of the Toutle River valley. Erosion flows stand up and interflows do not {figure and_mass wasting have cut into the flows and 25). The slopes tend to be more eroded than 36 Forest Slope Stability Project

Figure 25.-Stereo pair of scarp slopes east of Signal Peak. Note the flows dipping into the hillside.

Dvd, and debris avalanches and/or torrents elastic interbeds may erode, undercutting more are more common. Slopes are stable up to 15°, resistant basalt or andesite flows (which moderately stable from 15 to 30°, and unstable usually form free faces), causing rockfalls over 30°. or debris avalanches. Slopes up to 20° are usually stable, 20 to 30° moderately stable, OHANAPECOSH TERRAIN and 30+ 0 are unstable.

The third terrain group is the Ohanapecosh BEDROCK TERRAIN landforms. These are the relatively smooth bedrock controlled slopes in the eastern half The fourth terrain group includes two of the South Fork of the Toutle River drainage miscellaneous bedrock landforms - a plutonic where basalt and andesite flows and volcani­ intrusion and a recent lava flow on the west clastics generally dip eastward (figure 26). flank of Mount St. Helens. Stability is generally dependent on bedding attitude and slope. Ohanapecosh dip slopes (Od): Dip slopes are normally straight and smooth with angles of dip decreasing to the east. Dissection is much less than the landforms to the west. Stability is dependent on dip or slope angles. Generally up to 15° is stable, 15 to 25° is moderately stable, and 25+ 0 is unstable. Debris avalanches on the steeper slopes are most common. Ohanapecosh scarp slopes (Ox): These Figure 26.-Dip (Od) and scarp slopes (Ox) of Ohanapecosh terrains Goat Mountain are the steeper valley sides that cut against intrusion (Bi), and older mud or or across the dipping formation. Weak volcani- pyroclastic deposits (Mp) . Southern Cascades-South Fork of the Toutle River Area 37

Lava flow (Bf): One recent lava flow on LANDSLIDE TERRAIN the flank of Mount St. Helens is in the South Fork of the Toutle River drainage, It is The fifth group includes landforms where stable except near the edges, where rockfalls erosion processes, particularly stream erosion, are possible. are very active. These landforms are the most Goat Mountain intrusion (Bi): A Tertiary unstable in the South Fork of the Toutle River intrusion makes up Goat Mountain. It is drainage and have a potential for contributing mainly a steep free face with talus slopes large amounts of sediment to the river. surrounding it. Rockfalls and snow avalanches Incised valleys (Li): This landform are common. embodies the geologic erosional process,

TABLE 5.-Generalized instablility of the South Fork of the Toutle River landforms

Landform Low Moderate Alluvial-glacial terrain Mudflow-alluvial valley (Mf) X Older mud flows or pyroclastic flows (Mp) X Terraces and benches (T/B) X Glaciated valleys (Glv) X Glaciated headwalls-valley sides (Glh) X X

Dissected volcanic terrain Dissected slopes (Dvs): O - 15° X 15 - 25° X 25°+ X Dip slopes (Dvd): 0 - 15° X 15 - 25° X 25°+ X Scarp slopes (Dvx): 0 - 20° X 20 - 30° X X Ohanapecosh terrain Dip slopes (Od): 0 - 15° X 15 - 25° X 25°+ X Scarp slopes(Ox): 0 - 20° X 20 - 30° X 30°+ X Bedrock terrain Lava flow (Bf) X Goat Mountain intrusion (Bi) X Landslide terrain Incised valleys (Li) X Eroding valley sides (Lv) X 38 Forest Slope Stability Project

TABLE 6.-Types of hazards associated with the South For>k of the Toutle River landforms

Landform E A D St R Rf C ~ Flood Alluvial-glacial terrain Mudf1ow alluvial valley (Mf) X X X Older mudflows and pyroclastic flows (Mp) X Terraces and benches (T/B) X Glaciated valleys (Glv) X X Glacial headwalls-valley sides(Glh) X X Dissected volcanic terrain Dissected slopes {Dvs): O - 15° X X X 15 - 25° X X X X X 25°+ X X X X X X X Dip slopes (Dvd): O - 15° 15 - 25° X X X 25°+ X X X X Scarp slopes (Dvx): 0 - 20° X X 20 - 30° X X X X 30°+ X X X X Ohanapecosh terrain X Dip slopes (Od) 0 - 15° X 15 - 25° X X X X X 25°+ X X X X X X X Scarp slopes (Ox) O - 20° X X 20 - 30° X X X X X X 30°+ X X X X X X X Bedrock terrain Lava flow (Bf) X X Goat Mountain intrusion {Bi) X X X Landslide terrain Incised valleys (Li) X X X X X X X Eroding valley sides (Lv) X X X

E - Slump-earthflow St - Stream erosion C - Snow avalanche A - Debris avalanche R - Rockslide Dep - Deposition of mass wasting adjacent hill slopes D - Debris torrent Rf - Rock fa 11 Flood - Stream flooding

stream erosion. Second- and third-order are usually steep and unstable. streams are in the active process of down­ Eroding valley sides (Lv): These are the cutting and valley widening. All along these very steep sides of the South Tou"':le River areas, undercutting by stream erosion causes banks near the headwaters, where active erosion surficial slumping and avalanching into the is occurring. Some areas have large pumice stream. They are areas where increased deposits that are very easily eroded and are runoff resulting from forest management in an almost continual state of movement; practices will cause increased erosion and vegetation is unable to reestablish itself sedimentation of the rivers. The streambanks because of the rapid erosion of these steep Olympic Peninsula-Clearwater River Area 39 slopes. Any undercutting or overloading will thick peripheral band of lower to middle accelerate the erosional process. Debris Eocene submarine basalt flows (Crescent For­ avalanches are common. mation), with a strip of middle Tertiary folded sandstones, siltstones, and conglom­ GENERAL INSTABILITY AND HAZARDS erates (Clallam, .Astoria, Lyre, and Ald1,1ell Formations, Twin River Group, and other Tables 5 and 6 summarize the stability sedimentary assemblages} parallel to the and types of mass movements occurring on each Strait of Juan de Fuca and on the south landform. Generally the landforms most sus­ (Tabor and Cady, 1978). Shifting crustal ceptible to mass movement are: plates piled up the marine sediments, forming (1) Incised valleys (Li} where under­ the Olympic Mountains. The collision of the cutting by streambank erosion causes slump­ eastward-moving Juan de Fuca plate and the earthflows, debris avalanches, and possibly North American plate bent, folded, and rockfalls. sheared the marine sediments as the rocks (2) Eroding valley sides (Lv) where were smashed together. large areas of pumice and soil are actively The Olympics were covered by an ice cap eroding off the slopes and vegetation has during the Pleistocene, and valley glaciers difficulty reestablishing itself. extended to the ocean on the west and to the (3) Glacial headwall {Gh) where steep Puget Lowland to the east. The northern edge precipitous slopes and rockfalls are common. of the peninsula was also covered by contin­ (4) Scarp slopes (Dvx and Ox) that are ental ice emanating from British Columbia. over 25° which are susceptible to debris Deep incision by rivers and streams and glacial avalanches and debris torrents. scouring formed a rugged core with very high Most areas of the southern Cascades are relief. Wave action and glacial sediments very similar to the South Fork nf the Toutl e formed a coastal terrace area with low relief. River area in rock types and landforms. Glaciation and active river downcutting have Local variation in degree of weathering, depth removed most weathered materials. Thin of tephra deposits, and erosion should not glacial soils are common on the coastal plain, prevent extrapolation to other southern Cascade and thin colluvial soils and active mass drainages. One exception is the area along wasting dominate the steeper slopes. Only the Columbia River Gorge where massive land­ along the very southern foothills of the slides resulting from very weathered or altered Olympic Mountains are deeoer residual soils volcaniclastic interbeds have caused massive found. landsliding. For a summary and distribution Precipitation varies dramatically on the of these slides see Palmer (1977). Olympic Peninsula. In the interior of the mountains annual rainfall up to 500 cm (200 OLYMPIC PENINSULA-CLEAR\~ATER RIVER AREA in) is common, but to the northeast near Sequim rainfall can be as little as 43 cm The Olympic Peninsula has a core of (17 in) annually. Rainfall intensities slightly to moderately metamorphosed upper range from 20 cm/hr (8 in/hr) for a 2-yr, Eocene and Oligocene marine sediments (Hoh 24-hr storm in the interior to 5 cm/hr and western lithic assemblages). It is (2 in/hr) for the same storm along the north­ bordered on the north, east, and south by a east margin of the peninsula. 40 Forest Slope Stability Project

The Clearwater River is located on the so deformed that they are treated here as west side of the peninsula, in western essentially one large mass of marine sediments. Jefferson County, and is a major tributary of Structural and lithologic control of the the Queets River (figure 27). The basin in­ physiography is not greatly evident because cludes areas of sharp, high relief in the of the general homogeneity of the rock type. headwaters and lower relief along the coastal During the Pleistocene, glaciers flowing plain. Our study area covers only the upper down the Queets Valley and into the headwaters third of the drainage, where high relief and of the Solleks and Clearwater drainages high rainfall are dominant. scoured the valleys into a characte.ristic U­ The geology of the Clearwater River shape approximately 5 km (3 mi) down the study area has not been mapped in detail, but Clearwater and 3 km (2 mi) down the Solleks. the rocks are thick-bedded lithic graywacke Till from these glaciers is found mantling and thin- to thick-bedded siltstone, which some of the valley sides in these areas. Ice break down into soft chips, sand, and silt. from the Hoh glacier flowed into the Clear­ The strata are "complexly folded, faulted water drainage in the vicinity of Nancy Creek, and sheared, with the intensity of shearing damming the Clearwater. A terrace of silt increasing eastward" (Stewart, 1970, p. 12). and sand on the north side of the valley, The complexity of folding, faulting, and upstream from Nancy Creek, is made of depo­ shearing is often seen in the field by changes sits from this ice-dammed lake. Fluvial of lithology, strata attitude, and degree of gravel terraces are also located near the weathering occurring within a few meters mouths of the Solleks River and Stequaleho distance. The rocks within the drainage are Creek.

R IIW RIOW R9W

T26N

T2!5N CLEARWATER RIVER SCALE MILES

0 4 8

KILOMETERS

Figure 27 .-Map of Clearwater River study area. Olympic Peninsula-Clearwater River Area 41

Logging began in the Clearwater study sediment to the rivers and streams (Fiksdal, area about 1966. The road system is now 1974). There are fewer road-related mass almost complete, and roads and clearcuts wasting events in the upper reaches of the border the Olympic National Park. The Clear­ drainage, either due to better road engineer­ water basin is currently a very intensively ing and maintenance, fewer roads and better managed area and is a very valuable source locations, or different logging methods than of old-growth timber for the state, which in the lower reaches. owns most of the area. The relationship of bedrock geology to Mass wasting in the Clearwater River mass wasting in the Clearwater basin is not basin is strikingly different from that in very clear. Because detailed bedrock mapping the other four study basins. Failure consists does not exist, one cannot differentiate almost totally of debris avalanches and stream unstable or stable strata. However, it is incision. Only in the downstream portions of clear from field investigations that many the Clearwater, where fine-grained glacial shear zones are present, and that they tend deposits form terraces, are there many slump­ to be less competent than the usual bedded ea rthfl ows. or massive sediments found in this region. The Clearwater basin appears to be in a Structural dip influences stability, but the very youthful stage of development, with head­ structure of the area is so chaotic that dip ward erosion causing oversteepening in head­ is generally impossible to determine. water areas. The debris avalanches mapped It is also very difficult to determine the represent only about 50 percent of all the hydrologic conditions. Springs occur almost debris avalanches. The rest were too small everywhere. The major unstable areas are to be seen through the forest cover or were heads of drainages where springs surface, obliterated when logged. Most heads of undermining and oversteepening slopes. drainages in this area are probably continu­ Because of the extraordinary amount of rain­ ously active. fall in this region, it is not uncommon for Undercutting of hill slopes by the bursts of ground water to suddenly surface on streams and rivers is nearly ubiquitous. hill slopes and cause debris avalanches. Only the larger 3rd- and 4th-order streams In the upper and lower reaches of the and the upper Clearwater and Solleks Rivers Clearwater basin, glacial action has modified (through glacial modification) have developed the shape of the valley and mantled the lower floodplains. The 1st-, 2nd-, and 3rd-order slopes with till. Erosion has stripped most streams are almost continuous zones of of the till away, but now pockets of till or streamside undercutting, which often triggers clay are commonly found when a slope is cut debris avalanching upslope. •for a road, and these pockets cause slope The mass wasting maps show conclusively instability. that with development of roads into the The instability in the Clearwater area drainage, fill (predominantly sidecast) and is basically a net result of high rainfall cut failures have had a great impact on the and very active headward erosion and stream area. Sidecast failures are very common incision. The majority of slopes are at the throughout the Clearwater basin (especially in angle of repose, and any disturbance may the lower regions where logging was started in upset the balance among the geologic and the mid 1960's) and are a major source of hydrologic factors. The critically unstable Forest Slope Stability Project

are the over-steepened headwalls of the Only one fan large enough to map was found undercut streamside in the middle reach of the Solleks River. It is downslope from the largest debris The Clearwater study area has the fewest avalanche in the Clearwater study area. It (and probably the simplest) landforms of our is stable, as are the smaller fans, except five drainages. Because the bedrock geology when undercut. is uniform (in a broad sense) and structure Glaciated valleys (Glv): The upper is not really discernible, the land surface reaches of the Clearwater and Solleks Rivers is not very complex, except for some glacial have the typical glacial U-shape. Till and modifications. About 80 percent of the study gravel now fill the valley bottoms, and area is mapped as one landform (dissected because of the low slope gradients (up to bedrock hill slopes), and the rest of the 15°) stability is not a problem unless these drainage is divided into fluvial, glacial, deposits are undercut. Pockets of clay and and erosional landforms. till can become very unstable and cutbanks may collapse. ALLUVIAL-GLACIAL TERRAIN Glacial slopes (Gls): In the upper Clear­ water, Solleks, and Stequaleho basins, the The alluvial landforms were formed pri­ lower slopes of the main river valleys have marily by depositional processes, while the been scoured by glacial movement (figure 28) glacial landforms were derived from both and mantled with till and in some places clay. depositional and erosional processes. Glaciers Since glaciation, these slopes have been highly occupied the upper ends of the Clearwater and dissected and eroded, but till and clay pockets Solleks Rivers, and overtopped the ridge into remain. There is no way to predict where these the Stequaleho from the Queets drainage, where pockets occur, except possibly by site evalu­ they occupied a small area near Queets Ridge. ation. When these pockets of till or clay Floodplains (F): The floodplain is a are encountered, they will probably be un­ small area near the junction of the Solleks stable; otherwise the landform is moderately River and Stequaleho Creek and about four miles stable. above Nancy Creek. The materials are alluvial sand and gravel, and have little or no slope. The floodplains are stable but may undergo periodic flooding. Terraces (T): Gravel terraces are located primarily near the confluence of the three major drainages.in this study basin. They are flat topped and composed of sand and gravel. This landform includes terrace scarps that are not being actively eroded. The terraces are stable except when terrace scarps are undercut. Alluvial fans (Af): Many small unmap­ Figure 28.-U-shaped glacial valley in the pable fans of debris avalanche or debris torrent upper Clearwater River with glacial deposits are found at the mouths of 1st- and valley (Glv), glacial slopes (Gls), and dissected bedrock slopes (Bs) 2nd-order streams in the Clearwater area. outlined. Olympic Peninsula-Clearwater River Area 43

Glacial terraces (Glt): Some terraces were too small to map at l :24,000 scale. found in the Clearwater (figure 29) and Solleks Dissected slopes (Bs): Downcutting and drainages are composed of glacial sediment headward erosion throughout the study area rather than alluvial gravels. These terraces have formed a large, basically homogeneous are predominantly lacustrine silts, probably landform that is composed of moderate to very deposited in ice-dammed lakes. Many large steep (up to 50°) slopes. Much of this land­ slump-earthflows were identified at the edges form is constantly undergoing progressive of these terraces, where stream erosion has erosion by stream undercutting, headward undercut the lacustrine sediments. Because of erosion, and mass wasting. Locally, there the low gradient (up to 10°} the surfaces of are stable areas where bedrock is competent these terraces are stable, but when undercut and/or slopes are less than 20°. Unstable the cuts may become very unstable. areas include the heads of almost every 1st­ order stream, where erosion is actively BEDROCK TERRAIN undercutting the head of the drainage; and where downcutting by the 1st-, 2nd-, and 3rd­ Bedrock landforms are controlled by ord€r streams oversteepens and erodes hill underlying rock and structure, and make up slopes. The only generally stable slopes approximately 85 percent of the drainage. are those under 10°, slopes between 10° and Ridge tops (Bt): These landforms are the 25° are moderately stable, and unstable tops of rtdges and their associated gentle (25°+) slopes are by far more common. slopes. They are topographically above 1st­ Depending on forest management practices, order streams, so little dissection has slope stability may or may not be significantly occurred. Slopes range up to 15° and are altered. Undercutting, filling, or sidecasting stab 1e. by road building may alter the natural stability Dissected gentle slopes (Bg): These are in most areas of this landform, especially on the gentle bedrock slopes (less than 10°) on the steeper slopes. Because rock type and which mass wasting is not prevalent. They structure are so variable, site-specific evalu­ may have steeper unstable portions which ations may not always identify unstable areas.

Figure 29.-Clearwater River valley east of Nancy Creek. Indicated are glacial terrace (Git), glacial slope (Gls}, terrace (T}, and dissected bedrock slopes (Bs). 44 Forest Slope Stability Project

LANDSLIDE TERRAIN Forest management practices can affect this type of erosional process by increasing As in the other drainages, stream erosion runoff, redirecting drainage, and changing accounts for a large part of sediment contri­ channel configurations. These activities buted to the streams, therefore this last land­ may cause an increase in stream erosion by form is important particularly in terms of increasing peak flows and overloading small forest management. drainages with road runoff. Incised valleys (Li): These landslide­ prone incised valleys are the usually narrow, GENERAL INSTABILITY AND HAZARDS V-shaped streams that are actively eroding their banks (only 2nd-, 3rd-, and 4th-order Tables 7 and 8 summarize the stability streams are mapped). The mass wasting maps in­ ratings and mass movement potential for each dicate the banks instability by the numerous landform in the Clearwater drainage. The stream failures found throughout the basin. landforms most susceptible to mass wasting are: Almost all streamsides show some evidence (1) Incised valleys (Li), in which there of past failure. Most stream failures are is constant erosion of streamsides and under­ debris avalanches. Usually only surficial cutting of streambanks. material flows into the streams, but in areas (2) Dissected bedrock slopes (Bs), in where unconsolidated deposits (particularly which oversteepened and saturated heads of the fine-grained lacustrine sedi~ents) are drainages are the source areas of most debris undercut, slump-earthflows may occur. avalanches. Stability then is related to degree of under­ (3) q1acial slopes (Gls) where pockets cutting and competence of the streambank. of clay and till may be encountered.

TABLE ?.~Generalized instability of CleaY'Water River landfoY'ms

Landform Moderate Alluvial-glacial terrain Floodplains (F) X Terraces (T) X Alluvial fans (Af) X Glaciated valleys (Glv) X Glacial slopes (Gls) X Glacial terraces (Glt) X

Bedrock terrain Ridge tops (Bt) X Dissected gentle slopes {Bg) X Dissected slopes (Bs) X (X)

Landslide terrain Incised valleys (Li) X

(X) Indicates active landslides or unfavorable slope or hydrologic conditions. Willipa Hills-Grays River Area 45

TABLE 8.~Types of hazards associated with Clea~ater River landforms

Landform E A D St R Rf C ~ Flood Alluvial-glacial terrain Floodplains (F) X X X Terraces (T) X X Alluvial fans (Af) X X Glaciated valleys (Glv) X X Glacial slopes (Gls) X X X Glacial terraces (Glt) X X X X

Bedrock terrain Ridge tops (Bt) Dissected gentle slopes (Bg) X X X X Dissected slopes (Bs) X X X X X X

Landslide terrain Incised valleys (Li) X X X X X

E - Slump-earthflow Rf - Rockfa 11 A - Debris avalanche C - Snow avalanche D - Debris torrent Dep - Deposition by mass wasting from St - Stream erosion adjacent hill slopes R - Rockslide Flood - Stream flooding

The three groups of rocks outside the torrents. These rock types and mass movement Clearwater drainage but relevant to the Olympic types also occur in the Grays River basin. Peninsula are highly weathered or steenly Along the margins of the peninsula dipping sedimentary rocks, basalt flows, and continental and alpine glacial sediments are continental glacial sediments. South of the widespread. Older Pleistocene deposits are Olympic core, highly weathered sedimentary more weathered and cause considerable numbers rocks (Lincoln Creek, Astoria, and Montesano of landslides and instability particularly Formations) may have mass wasting problems along Hood Canal. In other areas, older as where relief or undercutting affect stability. well as recent glacial deposits (especially To the north and east of t~e Olympic core the clays) are unstable when undercut. For the Crescent Formation basalt flows form steep geology and slope stability of the Hood Canal slopes where debris avalanches and debris region see Birdseye (1976a, b), Carson (1975; torrents are common. To the north, folded 1976a, b), Gayer (1976a, b), and Hanson sediments of the Lyre, Aldwell, and Clallam (1976a, b). Formations, and Twin River Group may behave similarly to other bedded sedimentary rocks. WILLAPA HILLS~GRAYS RIVER AREA Bedding dip may be parallel to slope, increas­ ing the potential for slump-earthflows and The rocks of the Willapa Hills are all planar mass movements, and scarp-slopes would of Tertiary age (less than 65 million years be more susceptible to debris avalanches and old); major types include submarine basalt 46 Forest Slop,e Stability Project flows, pillow basalts, breccias, and inter­ Precipitation in this region varies bedded sediments (Crescent Formation), and markedly with elevation and longitude. subareal volcanic flows, breccias, and re­ Annual rainfall rises from 180 cm (70 in) on lated sediments (Goble Volcanics), and mica­ the coast to 300 cm (120 in) on the highest ceous, tuffaceous, and carbonaceous marine hills, then declines sharply to about 100 cm and nonmarine sandstones and siltstones (40 in) in the Centralia-Chehalis basin. (Cowlitz, Lincoln Creek, and Astoria Forma­ The 2-yr, 24-hr storm intensity is 9 to 14 tions, Wolfe and McKee, 1968, 1972). In cm/hr (3.5 to 5.5 in/hr) over most of the addition, flood basalts (related to the province, but as low as 5 cm/hr (2 in/hr) Columbia River Basalts) cap ridges along in the east. the Columbia River and in the northeast. The Grays River basin is in the south­ Unconsolidated sediments occur as marine central part of the Willapa Hills province, terraces near \~illapa Bay and Grays Harbor in Wahkiakum and Pacific Counties. Drain­ and form thick fills in the larger valleys. age is generally west and southwest toward Many normal faults crisscross the area, the Columbia River (figure 30). The basin juxtaposing units and resulting in many includes relatively high, up to 850 m changes in rock type over short distances. (2800 ft), steep hills in the north, a band The Willapa Hills province has not been of very subdued topography across the middle, glaciated, so most of the province has been and somewhat higher, yet rounded hills cross exposed to weathering for over 10 million the southern edge, parts of which are made years. Very thick weathering profiles have up almost completely of flow topography. developed, except where mass wasting has The physiography corresponds very well to removed the soil. However, the lowering of the rock types: the steep ridges are formed sea level, which accompanied world-wide on basalt flows and breccias of the Crescent glaciation, caused downcutting of coastal Formation, the more subdued topography on streams, including the Columbia, with the sandstone and siltstone of the Cowlitz and resultant incision and undercutting of slopes Lincoln Creek Formations, and the slumped in the hills. Though sea level is again high hills on interbedded flows and sedimentary and the lower valleys are filled or filling rocks of the Goble Volcanics. with sediment, streams may still be over­ Structure also controls landform steepened in their upper reaches. The weak development in this basin. The dip of the nature of the rocks, combined with heavy rock layers is generally to the south, so rainfall, has produced an area in which land­ shallow dip slopes ordinarily occur on the sliding is a major mechanism of erosion and south sides of ridges, while the north sides landform development. Entire drainages may have steeper scarp slopes that cut across be characterized as "flow topography," or the bedding planes. The many faults have hummocky ground in the process of very slow broken up the rocks, weakening them and earthflow. There are also many examples of allowing the penetration of water that aids disjointed and pirated drainages. Relief is in weathering. Because of this, many streams high in the center of the hills but is are located along faults. generally moderate to low around the peripheral The lower reaches of the Grays River edges. were deeply incised during the last glacia- Willipa Hills-Grays River Area

R R7W

SCALE. MIL.ES .. KILOMETERS + GRAYS RIVER

T // N

TION

Figure 30.-Map of the Grays River study area. tion. This incision probably reached at Landforms in the Grays River basin are least into the central part of the basin, classified into five terrain groups. The causing undercutting and activation or re­ first of these was formed by stream processes; activation of slumps on the main stem and three have been shaped by hill slope processes the West and South Forks. Many of the larg­ acting on different kinds of bedrock (Crescent er slumps may date to over 12,000 years ago. volcanics, bedded sedimentary rocks, and Goble Landslide damming may also have occurred on volcanics). The last unit includes large the tributaries, and stream piracy has slump-earthflow features not included in changed the drainage area at various times. the other groups, and incised valley sides. With the postglacial rise in sea level, the Grays River has filled most of its drowned ALLUVIAL-GLACIAL TERRAIN valley, and meanders across this fill from the main stem-West Fork confluence to Grays Floodplains (F): This unit includes the Bay. wide, flat valley floors of the Grays River Most of the eastern half of the study in its tidewater reach (main and West Forks), was logged early in the century and now plus small sections of valley floor on the supports second-growth timber. Much of the main fork, South Fork, and West Fork (prob­ rest of the basin has been roaded and ably in areas dammed by landslides and later logged recently. A few old growth areas partly filled with alluvium). The materials remain, primarily in the northeast and near are gravels, sands, and silts deposited by the South Fork. the streams. There is no instability within ll8 Forest Slope Stability Project this zone, but some hazard from slides which large blocks quickly break down into cobbles may occur on adjacent slopes. and pebbles. Weathering zones extend 2 to 3 Tributary valley (V): Relatively flat m (6 to 10 ft) below the surface, but soils valley bottoms, on the order of 10 to 100 m are very thin (normally less than 0.5, m). (30 to 350 ft) wide, have been formed in the Dissected topography (Cd): Debris upper reaches of many of the small streams avalanche and incision by tributary streams in the basin. These areas commonly slope up­ have been the dominant processes in the ward and merge with the hill slopes, due to evolution of the steep, ridge-and-valley the deposition of slopewash material from topography of this unit (figure 31). The above. These valleys are mostly cut into slopes are steep (20 to 50°), straight to bedrock, and little or no alluvium has been slightly convex, and generally short. deposited in them. As in the floodplain unit, Drainage density is high, especially in the the slope stability hazard comes from adja­ northwest. Combined with weak, low-cohesion cent (commonly oversteepened) hillsides, and regolith, these factors result in a high from debris torrents born in smaller tribu- · frequency of debris avalanche. Slopes of tari es. 30° are marginally unstable, and those Terraces-benches (T/B): Two large greater than 35° are likely to become un­ terraces at the main fork-West Fork con­ stable when logged or roads are built. fluence and scattered smaller fluvial ter­ Poorly compacted road fills may fail at 30°. races and landslide benches are included The short slopes allow the debris easy in this unit. The large terraces are flat, access to streams, filling channels and and formed on old deposits of alluvium resulting in debris torrents. (weathered gravel, sand, and silt); most of the other terraces are cut into bedrock, with little or no alluvium, and slope gently towards the streams. Landslide hazard in these zones may be caused by mass movement from adjacent uplands, and by failure into nearby streams.

CRESCENT TERRAIN

The first of the bedrock-controlled landform groups is the Crescent terrain, Figure 31.-Crescent dissected terrain. View formed on basaltic pillow lavas, lava flows, from West Fork of Grays River, Note dikes, breccias, and minor pyroclastic steep ridge-and-valley topography. deposits of the Crescent Formation, and on some breccias and siltstones of the Cowlitz Scarp slopes (Cx): In the northeastern Formation. Most of the rocks in this unit corner of the basin, the bedding of the are pervasively fractured, and may act almost Crescent exerts somewhat more control over like a coarse, cohesionless material. The the topography. The southward dip has pro­ southward dip of the unit does not seem to duced dip slopes on the south sides of cuesta have much influence on instability. The ridges, and steeper scarps on the north sides. Willipa Hills-Grays River Area 49

Long Ridge is the best example of this form. In general, these rocks were never strongly The scarp slopes are steeper (about 30 to 50°), cemented, and have since been deeply weathered; straighter, and shorter than most of the the glassy materials of the tuffaceous units Crescent terrain and do not have the extensive have been altered to clay. Jointing is per­ spur systems common to the Cd unit. There is vasive and random, and the rocks tend to break high potential for both debris avalanche and into small chips on exposure, and then into slump on these slopes, and the debris almost silt- and clay-sized grains. Soils are about always travels directly into streams. 0.5 to 3 m (1.5 to 10 ft) thick and fairly cohesive. The southward dip of the bedding BEDDED SEDIMENTARY TERRAIN exerts some (but not dominant) control over mass wasting and hill slope form. Hill slopes formed mostly on bedded rocks Moderate slopes (Bm): Hill slopes with (excluding the Goble Volcanics) have been com­ gradients on the order of 15 to 30° constitute bined into the next terrain group. Most of this subunit. Most of this landform type is these rocks are marine siltstones and sand­ in the eastern and central parts of the basin, stones, though the unit also contains some generally adjacent to the higher, steeper breccias, basaltic sills, and volcanic rocks. Crescent terrain (figure 32). Most of these

Figure 32.-Stereo pair of moderate slope (Bm) and gentle topography (Bg) of bedded sedimentary terrain. 50 Forest Slope Stability Project slopes are longer and more convex than the (less than 15°), low drainage density, and Crescent slopes, and drainage density is relatively thick regolith. Mass movement lower. Slump-earthflow is much more common features are uncommon in this unit, and in the cohesive regolith, and debris avalanches potential for instability is considered are confined to the steeper slopes. Breccias small. are the most unstable rocks. Road-related failures (fill and back-cut failure, dry GOBLE TERRAIN ravel) are common. Scarp slopes (Bx): Several cuesta The interbedded lava flows, pyroclastics, scarps have been mapped in the east-central siltstones, and sandstones of the Goble part of the basin. Like the Crescent scarp Volcanics are so susceptible to mass failure slopes, these slopes are steeper (up to 40°), that they are defined as a separate terrain shorter, and straighter than the dip slopes group. This terrain, a band of uplands across on the opposite sides of the ridges. As a the southern part of the basin, is made up result, they are more prone to avalanche (and mostly of overlapping and adjacent slump­ slightly more prone to slumping), although earthflows and flow topography; "normal" there has been little failure on the well­ slopes are uncommon. vegetated slopes. Most of the Goble Volcanics consist of Lincoln Creek topography (Bl): Tuffa­ massive subareal flows of basalt andandesitic ceous siltstone of the Lincoln Creek For­ basalt. These rocks are well-jointed and mation, which crops out on the southern break into blocks of gravel to boulder size. margin of the study basin (Fossil Creek), Parts of the flows were altered during forms a distinctive spur-and-swale topography. eruption, and weathering has occurred along The rocks are the youngest in the basin, and bedding planes and joints, so there are many have never been well-compacted or cemented; potential failure surfaces. Furthermore, they break down quickly into chips and grains. volcaniclastic materials (tuffs, breccias, and Internal drainage is poor, so there is high mudflow breccias) are interbedded with the potential for build-up of ground water, flows. These rocks are weaker than the lavas, leading to slope failure. The resultant and commonly fail under the weight of over­ topography is made up of fairly wide, convex lying rock. spurs, and steep (35 to 50°), short lower Regolith that formed on stable Goble slopes. Debris avalanches occur on slopes slopes is thin, 0.5 to 1 m (1.5 to 3 ft), greater than about 40 to 45°. Fossil Creek though on broken, more stable slump-earthflow and its tributaries downcut easily into the debris it may be many meters thick. soft bedrock, causing streamside slumping Slump topography (Gs): Most of the (most mapped as incision or slump terrain). Goble terrain consists of obvious slump­ Slump-earthflow is fairly common in this earthflow topography (figure 33). This unit, but slump topography is quickly eroded includes most of the mountain between the away. main stem and the West Fork, slopes around Gentle topography (Bg): South of the the South Fork, and the northern margin of East Fork, weak silts tone of the Cowlitz the Goble outcrop area. Incision by the Formation has been eroded into a relatively rivers is probably the most important local flat landscape. This area has low gradient control on slumping, but earthflow on south- Willipa Hills-Grays River Area 51

Figure 33.-Stereo pair of Goble slump topography. Cuesta scarp north of Blaney Mountain. ward dip slopes and foundering of Goble over since it is made of the same material as the weaker Cowlitz sediments have also been im­ Goble slump terrain. portant causes of mass movement. Most slopes consist of slump topography: arcuate head­ LANDSLIDE TERRAIN scarps (30 to 60° slopes), and hummocky, ir­ regular debris surfaces. Debris avalanches Areas in which mass erosion processes are common on the steeper slopes. Potential have been the dominant agents of landform for further mass movement in this landform development (excluding the Goble terrain) is generally high. Slopes near streams are are classified as landslide topography. This being reactivated by incision and many slumps terrain includes three units. on upper slopes are also active. Large areas Slump topography (Ls): Large slumps of slump debris with low slope gradient may have formed on all rock types. Some are be relatively stable if undisturbed, but the located high on hillsides, while others are broken material is weak, and may fail at on lower valley walls. Some of the slides relatively low angles if undercut. Any slopes are active, but most are dormant or stabilized. steeper than about 40° are subject to debris Headscarps are subject to debris avalanche and avalanche. progressive back-slumping. The slump debris Other slopes (Go): Areas not made up of is more stable, but may still be flowing obvious slump-earthflow features are mapped slowly. There is potential for reactivation in this unit; most of this landform type is of the landslides by stream erosion, construc­ tion activities, and large storms. located in the southeasternI corner of the basin. The landscapes includ~ hill s·lopes Flow topography (L.fl: Some areas have along the West Fork and Blaney Mountain, hurrunocky topography suggestive of earthflow, shallow dip slopes near the South Fork, and but without obvious headscarps or signs of some old flow topography. There is the recent movement. These have been mapped as potential for slump-earthflow in this unit, flow topography; they are all formed on 52 Forest Slope Stability Project sedimentary rocks. These subdued features are Fork, and many smaller streams. Most of the probably stable where undisturbed, but their material involved is weathered bedrock. The materials are weak and will probably not slopes, with gradients of 35 to 90°, commonly stand at very steep angles. have bare scarps, scalloped shapes, and Incised valleys (Li): Active downcutting slump debris (if it has not yet been removed and sidecutting by streams have formed large by the streams). These zones have high slope zones of mass wasting. There has been a instability potential, since the incision is great deal of stream-related failure in the still taking place, and the debris moves Grays River basin because of the weakness of directly into the streams. most of the rock materials, the large amount of rainfall and runoff, and glacial-age GENERAL INSTABILITY AND HAZARDS lowering of stream base level. Incision zones have been mapped where Tables 9 and 10 give generalized insta­ there is a definite break in slope between bility and hazards for the Grays River basin. upper hillsides and the lower, incision­ In summary, the most critical terrain units related slopes. The largest incision zones are: are where the main fork (figure 34) and (1) The Crescent terrain (Cd, Cx), in South Fork cross the outcrop zone of the which there is high potential for shallow Goble Volcanics; smaller zones have been debris,failure and direct access of debris formed by the Lvest Fork, the upper East into streams.

Figure 34.-Stereo pair of main fork of Grays River incised into Goble Volcanics. Note the numerous large slump-earthflows (E) and debris avalanches (A). Willipa Hills-Grays River Area 53

(2) The Goble terrain (Gs, Go), in which lain by Columbia River Basalt (along the slump-earthflow features are almost ubiquitous. Columbia River and smaller areas in the north); the edges of these surfaces are (3) The incised valleys (Li), in which undercut and fail by slumping, as is dis­ active undercutting is taking place and the cussed in the section on the southern debris falls directly into streams. Cascades, and in Palmer (1977). (4) Steeper parts of the moderate Sandstones and siltstones of the Astoria slopes (Bm), scarp slopes (Bx), and slump Formation occur in large areas throughout the topography (Ls), in which weak rocks may Willapa Hills. These rocks produce relatively fail by slump and/or avalanche. cohesionless soils and are subject to debris Two major rock units that crop out in avalanche on moderate to steep slopes. Large the Wi 11 apa Hi 11 s are not represented in the slump-earthflows occur, predominantly where Grays River basin. Large surfaces are under- the bedrock dips downslope.

TABLE 9.~Generalized instability of Grays River landforms

Landform Low Moderate Alluvial-glacial terrain Floodplains (F) X Tributary valleys (V) X Terraces-benches (T/B) X

Crescent terrain Dissected topography (Cd) X Scarp slopes (Cx) X

Bedded sedimentary terrain Moderate slopes (Bm) X (X) Scarp slopes (Bx) X (X) Lincoln Creek topography (Bl) X (X) Gentle topography (Bg) X

Goble terrain Slump topography (Gs) X Other slopes (Go) X

Landslide terrain Slump topography (Ls) X Flow topography (Lf) X Incised valleys (Li) X

(X) Indicates active landslides, or unfavorable slope or hydrologic conditions. 54 Forest Slope Stability Project

TABLE 10.~Types of hazaPds associated with GPays RiveP landfoPn?s

Landform E A D St R Rf C ~ Flood Alluvial-glacial terrain Floodplains {F) X X X Tributary valleys (V) X X X Terraces-benches (T/B) X X

Crescent terrain Dissected topography (Cd) X X X X Scarp slopes (Cx) X X X

Bedded sedimentary terrain Moderate slopes (Bm) X X X Scarp slopes (Bx) X X Lincoln Creek topography (Bl) X X X Gentle topography (Bg) X

Goble terrain Slump topography (Gs) X X X Other slopes (Go) X

Landslide terrain Slump topography (Ls) X X X Flow topography (Lf) X Incised valleys (Li) X X X X X X

D - Slump-earthflow Rf - Rock fa 11 A - Debris avalanche C - Snow avalanche D - Debris torrent Dep - Deposition by mass wasting from St - Stream erosion adjacent hill slopes R - Rockslide Flood - Stream flooding

SUMMARY 2. Slopes that are parallel to the dip of the underlying sedimentary or This study evaluated the slope stability volcanic bedrock, especially if the of five drainages representative of the dip is greater than 20°. different geologic, geographic, and climatic 3. Those areas that are actively being regions of western Washington. In summary, undercut by rivers, streams, or man's slopes are usually unstable under any of the activities. following conditions: 1. Oversteepened heads of drainages, 4. Almost any slope over 35°, including those areas that are generally sidecast and fills. steeper than the average adjacent Where there are combinations of any of these hill slope and where springs or conditions, instability is usually greatly seeps are common. increased. Summary 55

In this study we took a landscape abundant ground water, headward rather than a parametric approach because erosion, and slopes over 35° result of the general lack of detailed geologic in mass wasting. mapping throughout the study area and 3. Erosional terrain of incised valleys western Washington. The landforms we iden­ found in all of the five drainages. tified in this report are keyed to mass 4. Slump and flow landforms of the wasting processes to delineate areas that Grays, Green, and Nooksack drainages. may be more susceptible to mass wasting. Therefore to transfer the findings of this The five drainages studied in this report report to other drainages one must become were chosen to serve as examples of the north, familiar with the landforms and associated central, and southern Cascades of Washington, mass wasting processes. Once landform recog­ the Olympic Peninsula, and the Willapa Hills. nition is accomplished extrapolation to The geology, soils, climate, and topography neighboring areas can be made. However, it of each drainage is representative of its should be understood that these maps and corresponding geographic area, and this repre­ methods do not substitute for. on-site inves­ sentation of each study drainage to its sur­ tigations for specific site evaluations, but rounding region helps in translating this are designed primarily for planning purposes. work to adjoining drainages. It would be in These maps delineate areas where there is the best interest of forest land managers to need to be more concerned for instability, become familiar with the landforms presented rather than pinpointing sites~they are a in this report so translation to other areas planning tool. In almost any area, given may be accomplished. Forest land managers the right conditions, slopes may become un­ should also become familiar with the great stable, but through proper planning and im­ wealth of information that the Private Forest plementation many instability problems can Land Grading and State Soils mapping program be avoided. has gathered. These studies have generated Generally, the following are the areas considerable data in terms of soils engi­ where there should be most concern because neering such as cohesion, particle size dis­ of the unstable nature of the landforms: tribution, soil depth, parent material, l. Dip slope landforms (those slopes infiltration, etc. These data in many cases conforming with the direction and are much more site specific than is this angle of dip of the underlying study, and can be used for a multitude of bedded rocks) which may become purposes varying from engineering to agri­ unstable at relatively lower angles· culture. Identification of landforms that (greater than 20°) than other slopes are most susceptible to mass wasting and in the Nooksack, Green, Toutle, and knowledge of soil properties would greatly Grays River drainages. increase the chances of avoiding potentially 2. Heads of drainages in the Clearwater unstable areas, thus reducing detrimental dissected bedrock landform where environmental impacts. 56 Forest Slope Stability Project

REFERENCES CITED

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Birdseye, R. U., 1976b, Relative slope stability in east-central Jefferson County, Washington: Washington Division of Geology and Earth Resources Open-File Report 76-27, 1 map, scale 1:24,000.

Burroughs, E. R.; Chalfant, G. R.; Townsend., M.A., 1976, Slope stability in road construction: U.S. Bureau of Land Management, Portland, Oregon, 102 p.

Carson, R. J., 1975, Slope stability map of southern Hood Canal area, Mason County, Washington: Washington Division of Geology and Earth Resources Open-File Report 75-4, 1 map, scale 1:62,500.

Carson, R. J., 1976a, Preliminary geologic map of the Brinnon area, Jefferson County, Washington: Washington Division of Geology and Earth Resources Open-File Report 76-3, l map, scale 1:24,000.

Carson, R. J., 1976b, Relative slope stability of the Brinnon area, Jefferson County, Washington: Washington Division of Geology and Earth Resources Open-File Report 76-15, 1 map, scale 1:24,000.

Crandell, D. R., 1971, Postglacial lahars from Mount Rainier Volcano, Washington: U.S. Geological Survey Professional Paper 677, 75 p.

Durgin, P. B., 1977, Landslides and the weathering of granitic rocks . .!!!_ Coates, D. R., editor, Landslides: Geological Society of America, Reviews in Engineering Geology, v. III, p. 127-131.

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Gayer, M. J., 1976a, Geologic map of northeastern Jefferson County, Washington: Washington Division of Geology and Earth Resources Open-File Report 76-21, l map, scale 1:24,000.

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Hammond, P. E., 1963, Structure and stratigraphy of the Keechelus volcanic group and associated Tertiary rocks in the west-central Cascade Range, vJashington: University of t,Jashi ngton Ph.D. thesis, 264 p.

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APPENDIX I-GLOSSARY

Andesite-Gray to grayish-black, fine-grained volcanic rock, common in the Cascade Range. Blueschist-Foliated metamorphic rocks. Breccia-A rock composed of angular fragmental pieces. Cirque-An amphitheater-shaped, steep-walled recess in a mountain carved by glacial erosion.

Conglomerate-A rock composed of rounded water-worn fragments cemented together. Cordilleran Ice Sheet-A continental ice cap that covered a large portion of western North America. Dacite-A light-colored, silica-rich, extrusive volcanic rock. Dip-The angle at which strata is inclined from the horizontal. Dunite-An igneous rock consisting primarily of the mineral olivine. In Washington found only in the Twin Sisters Mountain. Foliated-A rock having parallel fabrics, laminated structure resulting from segregation of different minerals during metamorphism. Gabbro-A dark, coarse-grained plutonic rock. Geomorphology-A branch of geology that deals with the general configuration of the earth's surface and the nature, origin, and development of present landforms, and their relationship to underlying structures. Greenschist-A metamorphosed igneous rock that owes its color and fabric to abundant chlorite. Hummocky-Uneven, irregular surface. Intrusive-Pertaining to a rock formed by magma emplaced into pre-existing rock. Lacustrine-Pertaining to lakes or sediments formed in lakes. Lahar-A mudflow composed mainly of volcaniclastic material originating on the flank of a volcano. Lithic-A descriptive term for volcaniclastic or sedimentary rocks containing abundant fragments of previously formed rocks, also describes those fragments. Lithology-Description of rock, rock type, physical character of a rock. Mafic-General term for dark-colored igneous rocks composed chiefly of ferromagnesian minerals.

Metamorphic-Pertaining to metamorphism-Rocks altered in mineral and physical characteristics by heat and/or pressure. Metasandstone-Sandstone that has undergone partial metamorphism, having both sedimentary and metamorphic characteristics. Normal fault-A fault in which one block slips down relative to another block, caused by tension or pulling apart of two blocks. Appendix I 61

Phyllite~A metamorphic rock that has a silky sheen due to microscopic layering. Pillow basalt~A term used for basalts extruded under water that form rounded globular "pillow-like" structures that range in size from a few centimeters to a meter or more (1 in to over 3 ft). Pluton~An igneous intrusion. Pyroclastic~Rock material formed by volcanic explosions~detrital volcanic ejecta. Regolith~The loose, incoherent rock material and soil overlying bedrock. Rhyolite~A light-colored, fine-grained, extrusive rock. Serpentinite~A rock consisting almost totally of serpentine, usually having a green color and soapy feel. Strike~The horizontal direction, bearing, or trend of a structural surface or strata. Thrust fault~A fault caused by compressional forces causing one block to override another. Tuff~A compacted pyroclastic deposit of ash or volcanic fragments. Ultramafic~Igneous rocks having virtually no quartz or feldspar. Volcanic~Pertaining to the activities, structures, or rock types of a volcano. Volcanic breccia~A pyroclastic rock consisting of angular volcanic fragments. Volcaniclastic~Pertaining to a rock derived from pieces of volcanic material no matter what origin. 62 Forest Slope Stability Project

APPENDIX II~GEOLOGIC TIME SCALE

GEOLOGIC TIME SCALE SUBDIVISIONS APPROXIMATE AGE BEFORE ERA PERIOD EPOCH PRESENT, MILLIONS OF YEARS Holocene (Recent) Quaternary .015 Pleistocene CENOZOIC 2 Pliocene 6 Miocene Tertiary 25 Oligocene 36 Eocene 58 Paleocene 65 Cretaceous 135 MESOZOIC Jurassic 180 Triassic 230 Permian 280 Pennsylvanian Many epochs recognized 310 Mississippian PALEOZOIC 340 Devonian 400 Silurian 430 Ordovician 500 Cambrian 570

PRECAMBRIAN 3,500+