United States 

Department of

Agriculture

Forest 

Service

July 2017

Geology Report

Craggy Vegetation Management Project

Salmon/Scott River Ranger District, Klamath National Forest

Siskiyou County, California

Prepared By: Angie Bell

Klamath National Forest Headquarters

1711 S. Main Street, Yreka, CA 96097

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Table of Contents

Geology Report............................................................................................................................1

Introduction............................................................................................................................. 1

Methodology........................................................................................................................... 1

Analysis Indicators.............................................................................................................. 1

Measures............................................................................................................................. 1

Spatial and Temporal Bounding of Analysis Area ............................................................. 5

Affected Environment............................................................................................................. 5

Environmental Consequences................................................................................................. 7

Alternative 1........................................................................................................................ 7

Direct and Indirect Effects.............................................................................................. 7

Cumulative Effects.......................................................................................................... 8

Alternative 2........................................................................................................................ 8

Direct and Indirect Effects.............................................................................................. 8

Cumulative Effects........................................................................................................ 11

Alternative 3...................................................................................................................... 11

Direct and Indirect Effects............................................................................................ 11

Cumulative Effects........................................................................................................ 11

Comparison of Alternatives.................................................................................................. 11

Compliance with law, regulation, policy, and the Forest Plan ............................................. 13

Literature Cited ..................................................................................................................... 14

List of Tables

Table 1: Risk Matrix using Likelihood and Consequence to Assess the Risk Category of a Landslide 

Event in a 7th Field Watershed ..............................................................................................................5

Table 2: Landslide Likelihood under Current Conditions for the Analysis Watersheds..............................6

Table 3: Landslide Risk under Current Conditions for Analysis Watersheds ..............................................7

Table 4: Summary of GEO Model Outputs and Percent of Watershed with High or Moderate Severity 

Disturbance under Alternative 2 for Indirect Effects and Cumulative Effects...................................10

Table 5: Summary of GEO Model Outputs and Percent of Watershed with High or Moderate Severity 

Disturbance under Alternative 3 for Indirect Effects and Cumulative Effects...................................11

Table 6: Summary of GEO Results for 7th Field Watersheds under Alternative 1 with a Modeled Wildfire 

Event and Action Alternatives with a Modeled Wildfire Event for Zero to Five Years after Project 

Implementation ...................................................................................................................................12

Table 7: Percent of Watershed that is Likely to Burn with High or Moderate Soil Burn Severity for all 

Alternatives.........................................................................................................................................12

Table 8: Summary of Effects to Landslide Risk with and without Wildfire Scenario................................12

Geology Report Craggy Vegetation Management Project

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Geology Report

Introduction

The purpose of this report is to analyze the effects of the project alternatives on landslide rates 

and groundwater resources.

The Forest Service Manual Chapter 2880 (Geologic Resources, Hazards and Services) requires 

the assessment of the risk of safety, property and natural resources from both naturally-occurring 

and management-related landslides. The Klamath National Forest Land and Resource 

Management Plan (Forest Plan) directs the Forest to manage vegetation on unstable lands to 

maintain or enhance slope stability (Forest Plan Standard and Guideline 2-1). Project level 

review of the unstable lands is required to validate the current mapping (Forest Plan Standard 

and Guideline 2-2). Unstable lands are defined as active landslides, inner gorges, toe zones of 

dormant landslides and severely weathered and dissected granitic lands. These features are 

considered Riparian Reserves (Forest Plan Standard and Guideline - Management Area 10-2).

Methodology

Analysis Indicators

Landslide Risk. The Forest Plan guides projects to manage vegetation on unstable lands to 

maintain or enhance slope stability. Slope stability can be defined in terms of landslide 

risk. Landslide risk is the chance of effects of injury or loss as a measure of the probability and 

adverse consequences to safety, property or natural resources. Landslide likelihood is determined 

by geomorphic landform, disturbance, landslide modeling, and road density. Debris slide and 

debris flow processes are most likely to be influenced by the loss of root strength and 

interception as a result of timber harvest and prescribed burning versus deep-seated landslides 

which are not as vulnerable to vegetation changes. Deep-seated landslides such as the large 

dormant landslide features that exist in the project area are not likely to be reactivated as a result 

of the proposed activities (Benda et al. 2005, Reid 2010, Slaymaker 2001). Therefore, the focus 

of the landslide risk assessment is to analyze the indirect effects of the project alternatives on 

debris slide and debris flow processes.

Measures

Landslide Risk

A forest-wide goal is to promote slope stability on geologically unstable lands (Forest Plan, 

pages 4 to 5). Slope stability is indirectly affected by many land management practices. 

However, it is difficult to directly measure slope stability due to the stochastic (random) and 

complex nature of landslide processes in the Klamath Mountains. This analysis uses landslide 

risk as a proxy for slope stability. Landslide risk is a combination of the likelihood a landslide 

event may occur and the consequences of such an event.

Watershed Modeling

The risk assessment uses the cumulative watershed effects (CWE) model results for the GEO 

model. Although this model is labeled as a ‘cumulative watershed effects’ model, it does not 

measure cumulative effects as defined by the Council of Environmental Quality for NEPA 
Geology Report Craggy Vegetation Management Project

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analysis. Instead, it provides information on the effects of alternatives by adding the effects of 

various activities proposed for each alternative (i.e. road construction, landings, thinning, etc.) to 

result in a risk ratio for the process model. It also provides information on the added effects of 

past activities and events to provide a baseline risk ratio to which to compare the effects of 

proposed alternatives. For the GEO model the threshold of concern for the risk ratio is 1.0. A risk 

ratio greater than or equal to 1.0 is a yellow flag and calls for a closer look at mitigation 

opportunities. Cumulative Watershed Effects: the Abridged Version (Bell 2012) provides a more 

detailed description of the model.

The GEO model estimates the volume of sediment delivered to the mouth of a 7th field watershed 

due to all types of landsliding during a 10-year storm event. The coefficients in the mathematical 

equation were developed in the Salmon River basin and the model assumes the geomorphic 

landforms react identically regardless of elevation. The indirect effects of an alternative will be 

analyzed using the landslide volume estimates attributed to the alternative relative to the existing 

condition volume estimates. Changes to landslide volume between the different alternatives is 

apparent in the resulting risk ratio.

The GEO model was run for Alternative 1 (current condition) and Alternatives 2 and 3 for the 

proposed activities alone. It was also run under a wildfire scenario for Alternatives 1 and 3. We 

chose to model potential wildfire watershed effects for Alternative 3 and not Alternative 2

because the GEO results of Alternative 2 and 3 are relatively similar. Therefore, the amount of

time that would be required to run the model for both action alternatives was not warranted 

because of the small differences between these alternatives. The modeling results for Alternative 

3 under the wildfire scenario will be used as a proxy for the effects of Alternative 2 under a 

wildfire scenario. The modeling was generated using the FlamMap modeling for fire potential 

zero to five years after project implementation (see the Fire and Fuels report for detailed 

methods). The resulting flame lengths were used with the GEO modeling as a proxy for soil burn 

severity. It was assumed that flame lengths of less than four feet would have a low soil burn 

severity, between 4.1 and 8 feet would have a moderate severity and greater than 8 feet would 

have high severity. The purpose of combining a wildfire scenario and GEO results is to illustrate 

the benefits of the Alternative 2 and 3 treatments on watershed health over the long term. The 

detailed results of the GEO modeling can be found in the project record in the cumulative 

watershed effects analysis excel spreadsheet, available on the project website.

Mapping of geologic and soil resources has not been completed for the Rocky Gulch-Yreka 

Creek 7th field watershed (18010207050206) because of the small amount of public lands in the 

watershed. This mapping is a required input into the model so there are no model results for 

Rocky Gulch-Yreka Creek. The landslide likelihood reported in this analysis was estimated 

using Long Gulch as an analogy for the baseline. However, about 10 percent of Rocky GulchYreka Creek was affected by a high and moderate severity wildfire in August 2016 (the Grade 

Fire). The GEO model risk ratio was estimated using Long Gulch as a baseline with the 

estimated effects of the Grade Fire added; it was assumed that 200 acres burned with moderate or 

high severity and the watershed is non-granitic with moderate slopes (less than 65 percent).

Landsliding Likelihood

Landslide likelihood is determined by geomorphic landform, disturbance, vegetation condition, 

bedrock type, and groundwater conditions. The analysis uses existing field reviewed geomorphic 

mapping, field reviewed bedrock mapping and historical landslide information to determine the 
Geology Report Craggy Vegetation Management Project

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likelihood of landsliding. A majority of the west side of the Forest (in which the project area is 

located) is relatively unstable so the likelihood of having small localized landslides in any given 

7th field watershed during any storm is highly probable with or without management activities. 

This analysis is focused on determining the likelihood of a landslide event. A landslide event is 

defined for this analysis as a large scale, extensive episode resulting in several landslides that 

could interrupt ingress and egress (road access) or fluvial processes on 3rd to 5th order streams, or 

damage major infrastructure across a 7th field watershed. (Dai et al. 2002, Wise et al. 2004, AGS 

2007).

The likelihood of a landsliding event was analyzed at the 7th field watershed scale using a 

combination of the percent of the watershed that was harvested or burned within the last 10 

years, the GEO risk ratio, and the percent of unstable lands. The likelihood categories are 

summarized below: (definitions of the risk categories referenced below (low to very high: in 

italics) may be found following Table 1).

1. Almost Certain – A landsliding event is expected to occur even under an average storm 

event (two-year storm event). A watershed in this category meets at least two of the 

following criteria: 1) greater than or equal to 25 percent of the watershed had high to 

moderate disturbance (based on soil burn severity, silviculture or fuels reduction 

prescriptions) in the past 10 years; 2) the GEO risk ratio is greater than or equal to 1.5; or 

3) greater than or equal to 25 percent of area is designated as unstable lands (per 

definition in Forest Plan).

2. Highly Likely – A landsliding event will probably occur under an average storm event. A 

watershed in this category meets at least one of the following criteria: 1) greater than or 

equal to 25 percent of the watershed had high to moderate disturbance (based on soil 

burn severity, silviculture or fuels reduction prescriptions) in the past 10 years; 2) the 

GEO risk ratio is greater than 0.95; or 3) greater than or equal to 25 percent of the area is 

designated as unstable lands.

3. Likely – A landsliding event is likely to occur under a 10- to 20-year storm event. A 

watershed in this category meets at least one of the following criteria: 1) between 10

percent and 25 percent of the watershed had high to moderate severity disturbance (based 

on soil burn severity, silviculture or fuels reduction prescriptions) in the past 10 years; 2) 

the GEO risk ratio is between 0.75 and 0.95; or 3) between 10 percent and 25 percent of 

the area is designated as unstable lands.

4. Unlikely – Landsliding might occur under a 20- to 99-year storm event. A watershed 

meets this category if it is on the west side of the Forest and does not meet the criteria in 

above categories.

5. Rare – Landsliding is conceivable but only under a greater than or equal to100-year storm 

event. A watershed meets this category if it is on the east side of the Forest (with 

occasional exceptions).

Consequence Categories

The consequences of a landslide are based on the impacts to elements at risk. The elements at 

risk for this analysis are human safety, infrastructure, property, recreation and visitor use, and 

environmental resources. The consequences categories are summarized below:
Geology Report Craggy Vegetation Management Project

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1. Catastrophic Consequences – In this category human health and safety are susceptible 

to landslide events. The presence of occupied structures (homes, businesses, work 

areas), campgrounds, or heavily used roads that are vulnerable to (in the path of) a 

potential landslide event meets the criteria for this category.

2. Major Consequences – In this category essential infrastructure such as main National 

Forest Transportation System (system) roads (maintenance levels 3, 4 and 5), power 

lines, pipelines, municipal water sources, and railroads are susceptible to (in the path 

of) landslide events and may be rendered inoperative as a result. This category is also 

used to describe the vulnerability of anadromous fish habitat to landslide events that 

lead to the complete loss of their habitat. This category also applies when stream 

channels are vulnerable to landsliding which leads to aggrading (depositing sediment) 

or degrading (removing sediment) the stream channel. Also, the reduction of shade 

and riparian vegetation over a large portion (greater than or equal to 50 percent) of 

late flowing intermittent or perennial streams is a major consequence. See the 

hydrology and aquatics resource reports for more information regarding the impacts 

of these effects on shade.

3. Moderate Consequences – In this category only non-essential infrastructure and 

property such campgrounds (unoccupied), trailheads, day use areas and system trails 

are vulnerable to landslide events and will be rendered inoperable. Landsliding debris 

will temporarily block major ingress and egress roadways. Anadromous fish habitat is 

vulnerable to a partial loss or short-term impairment. This category also applies when 

stream channels are vulnerable to aggradation, degradation or a reduction of shade 

over less than 50 percent of late-flowing intermittent and perennial streams.

4. Minor Consequences – In this category infrastructure is vulnerable to damage that 

does not render it inoperable but makes its operation unsafe but repairable or 

inconvenient (e.g. debris partially blocking a two-lane road) as a result of landslide 

events. The minor damage category applies to landsliding into streams that are 

vulnerable to a reduction of shade at the site scale or blocking the stream for the short 

term (less than six months).

5. Nuisance Consequences – In this category there is no infrastructure, fish habitat or

stream shade vulnerable to landslide events.

Landslide Risk Matrix

The landslide risk matrix (Table 1) is the crosswalk between the likelihood, consequences, and 

the implications of landslide risk. Once the likelihood and consequences have been determined 

using the criteria above, the risk matrix is used to determine the risk category that fits that 

situation. For instance, you may have an area that is likely to experience landsliding during a 10-

year storm event and the essential infrastructure is susceptible to landsliding; in this case the 

landslide risk is high. This risk has implications for forest management which are described in 

the section below.
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Table 1: Risk Matrix using Likelihood and Consequence to Assess the Risk Category of a Landslide Event in 

a 7th Field Watershed

Consequences/Likelihood Almost 

Certain Highly likely Likely Unlikely Rare

Catastrophic 

Consequences Very High Very High High Moderate Moderate

Major Consequences Very High High High Moderate Low

Moderate Consequences High High Moderate Moderate Very Low

Minor Consequences High Moderate Low Low Very Low

Nuisance Consequences Low Low Very Low Very Low Very Low

The Risk Category Implications

1. Very High - Health and human safety or essential infrastructure is at risk. There is an 

immediate and urgent need to reduce the likelihood of landsliding or mitigate the 

consequence to the elements at risk.

2. High - There is a reasonable probability that landsliding will impact essential 

infrastructure and may impact health and human safety. Non-essential infrastructure as 

well as recreation and visitor use may be impacted. Project-wide and global mitigations 

need to be in place to minimize impacts to landslide processes for actions in these 

watersheds.

3. Moderate - There is a moderate probability of impacts to essential or non-essential 

infrastructure or health and human safety as a result of landsliding. The cost and benefit 

of mitigations needs to be considered before actions are proposed for implementation. 

Strategic or localized mitigations need to be in place to minimize impacts to landslide 

processes for actions in these watersheds.

4. Low - There a low probability of impacts to elements at risk. Remediation of landsliding 

consequences may be the most cost effective method of dealing with these areas.

5. Very Low - There is almost no probability of impacts to elements at risk as a result of 

landsliding. Mitigations are rarely needed.

Spatial and Temporal Bounding of Analysis Area

The spatial scale for landslide risk is the 7th field watershed scale for the direct and indirect 

effects and the cumulative effects. The models used for analysis are calibrated at a 7th field scale 

(Bell 2012). The temporal scale is zero to 10 years for short term and greater than 10 years for 

long term. Elevated landslide rates due to forest management have been shown to begin to 

decrease around seven to 12 years after disturbance in Northern California (Ziemer 1981).

Affected Environment

Landslide Risk
Geology Report Craggy Vegetation Management Project

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Upper Humbug and Middle Fork Humbug Creeks both have a highly likely likelihood of a 

landslide event occurring during an average storm event. This is mainly a result of the 

combination of high road densities and the steep, weathered granitic bedrock in the watersheds. 

Lower Humbug Creek, Clear Creek and Rocky Gulch are likely to have landslide events during a 

10- to 20-year storm event. These watersheds have less area underlain by steep, weathered 

granitic bedrock than Upper and Middle Fork Humbug Creek. Greenhorn Creek, Yreka CityYreka Creek, and Long Gulch are unlikely to experience a landsliding event even during a 20 

year storm event.

Table 2: Landslide Likelihood under Current Conditions for the Analysis Watersheds

Watershed Code Watershed Name

Existing 

GEO model 

Risk Ratio

Unstable 

Lands (% 

Watershed)

Watershed 

Disturbance 

(% 

Watershed)

Landslide 

Likelihood

18010206080101 Upper Humbug Creek 1.37 17% 0% Highly 

Likely

18010206080102 Middle Fork Humbug 

Creek 1.10 11% 0% Highly 

Likely

18010206080104 Lower Humbug Creek 0.52 13% 0% Likely

18010206080103 Clear Creek 0.61 16% 0% Likely

18010207050203 Greenhorn Creek 0.49 9% 9% Unlikely

18010207050204 Yreka City-Yreka Creek 0.29 1% 0% Unlikely

18010207050205 Long Gulch 0.59 1% 0% Unlikely

18010207050206 Rocky Gulch-Yreka Creek 0.75 0% 0% Likely

Upper Humbug Creek would have moderate consequences as a result of a landslide event. The 

values at risk include endangered fish habitat and the potential for damage to private property 

with a low probability of a landslide affecting an occupied residence. Middle Fork Humbug and 

Lower Humbug Creek would have major consequences as a result of a landslide event mainly 

because of the primary ingress/egress roadways that could be affected by debris flows in the 

watershed. Lower Humbug Creek has year-round residences along its banks that are vulnerable 

to debris flow events. Clear Creek would have moderate consequences because of the potential 

for damage to private property with a low probability of a landslide affecting an occupied 

residence.

Greenhorn Creek would have nuisance consequences because the effects are restricted to 

interruptions of recreation at the city park including recreational fishing and effects to nonessential infrastructure such as bridges on the walking path. Yreka City-Yreka Creek, and Long 

Gulch will have minor consequences from a landslide event. The stream channels in the Yreka 

City-Yreka Creek watershed have been modified to carry flood events and debris flows. Long 

Gulch flows into Yreka Creek downstream of the City of Yreka; a large floodplain in that area

provides defense against the effects of debris flows.

The Grade Fire burned with high and moderate severity in Rocky Gulch-Yreka Creek. This 

makes debris flow initiation locations less predictable because a landslide could be triggered on 

any portion of the fire area during an average storm event. Prior to the fire the primary landslide 

process was debris flow of material already in the stream channel. New debris sliding could 

occur anywhere in the drainage affected by moderate or high severity fire-generated debris flows 
Geology Report Craggy Vegetation Management Project

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from anywhere on the hillslope. This means that private land could be affected if a debris slide 

from the hillslope initiates a debris flow outside of existing stream channels. The most likely 

effect would be deposits of sediment on private property and superficial damage to private 

property. This means that the landslide consequence in Rocky Gulch-Yreka Creek is moderate.

Table 3: Landslide Risk under Current Conditions for Analysis Watersheds

Watershed Name Likelihood Consequence Risk

Upper Humbug Creek Highly Likely Moderate High

Middle Fork Humbug Creek Highly Likely Major High

Lower Humbug Creek Likely Major High

Clear Creek Likely Moderate Moderate

Greenhorn Creek Unlikely Nuisance Very Low

Yreka City-Yreka Creek Unlikely Minor Low

Long Gulch Unlikely Minor Low

Rocky Gulch-Yreka Creek Likely Moderate Moderate

Upper, Lower and Middle Fork Humbug Creeks currently have high landslide risks. Clear Creek 

and Rocky Gulch-Yreka Creek have moderate risks. Greenhorn has a very low landslide risk. 

Yreka City-Yreka Creek and Long Gulch and have low landslide risks under current conditions.

Environmental Consequences

Alternative 1

Direct and Indirect Effects

No treatment in the project area would lead to decreased tree vigor and in some stands increased 

tree mortality due to competition in the timber stands over time. Without treatment the species 

diversity of the timber stands would decrease, making the stand more likely to be impacted by 

highly intense and severe wildfires (see the Fire and Fuels report and the Silviculture report). The 

increased tree mortality would increase landslide likelihood by reducing root cohesion 

(Swanston and Dryness 1973) and precipitation interception (Reid 2010). Alterations to 

evapotranspiration and precipitation interception processes due to tree loss would also increase 

landslide likelihood across the watersheds. High severity wildfire, which is associated with tree 

mortality, would increase the landslide likelihood should an event occur.

If no wildfire should occur within the spatial and temporal boundaries of this analysis the 

landslide likelihood will decrease slightly overtime as the areas effected by past disturbances 

recover. This recovery outweighs the decreased forest health in the watershed for landslide risk 

reduction. One of the primary drivers of the high landslide risks in the Humbug Creek area is the 

high road density. The decision made regarding travel management reduced the administrative 

road density and the unauthorized route (previously non-system road) density; these roads and 

routes are passively recovering in the watershed. This would reduce the landslide risk over time. 

However, the risk would remain high for Upper, Middle and Lower Humbug because of the high 

road densities in the watersheds.
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Under a Wildfire Scenario

If a wildfire should occur in the project area and no treatment is implemented the GEO risk ratios 

will be increased to well over the threshold of concern of 1.0 for Upper, Middle, and Lower 

Humbug, Clear Creek, Long Gulch and Rocky Gulch-Yreka Creek. The model results for 

Greenhorn Creek and Yreka City-Yreka Creek are likely an overestimate considering great 

lengths would be implemented to prevent a wildfire from entering or spreading in these 

watersheds, however even with the overestimation they would remain close to or below the 

threshold of concern for the GEO model.

It is assumed in this analysis that a high intensity wildfire in any of the 7th field watersheds in the 

Humbug 5th field watershed are likely to result in at least 20 to 25 percent of the watershed 

having high or moderate soil burn severity. This is based on information from previous wildfires 

including the 2008 wildfires, the Salmon Complex Fires in 2013 and the 2014 wildfires. 

Greenhorn Creek, Yreka Creek and Long Gulch are likely to have a lower percent of high and 

moderate severity fire given the proximity to the city of Yreka, priority of firefighting resources, 

and access to the area. If 25 percent or more of the watersheds have high or moderate soil burn 

severity, this indicates that the landslide likelihood would increase to almost certain for Upper,

Middle Fork and Lower Humbug Creek and Clear Creek. Based on increases in the GEO model, 

Long Gulch will move into a highly likely landslide likelihood. Rocky Gulch-Yreka Creek will 

go to a highly likely landslide likelihood because of the current conditions created by the Grade 

Fire. If the model results are not an overestimate, Greenhorn will move to a highly likely

category and Yreka City-Yreka Creek to likely.

This would lead to landslide risk for Middle and Lower Humbug Creek moving from a high

landslide risk to a very high landslide risk. It would also lead to Clear Creek moving from a 

moderate landslide risk to a high landslide risk. Upper Humbug Creek would continue to have a 

high landslide risk even with a wildfire event. Greenhorn Creek would move to low. Yreka CityYreka Creek would remain at low. Long Gulch would move to moderate. Rocky Gulch-Yreka 

Creek would have a high landslide risk if the rest of the watershed were to burn as the fuels 

model predicts. To see the change in risk ratio for the analysis area under Alternative 1 with or 

without a wildfire event, see Table 6.

Cumulative Effects

The only action considered a reasonably foreseeable event that will have an effect on landslide 

risk is a timber harvest plan on private land in Lower Humbug Creek. The timber harvest plan 

covers a small area and will only increase the risk ratio in the watershed by 0.01. This is not 

enough to increase the landslide risk for the watershed. This means that the cumulative landslide 

risk remains the same for all watersheds in the analysis area as described in the direct and 

indirect effects section above.

Alternative 2

Direct and Indirect Effects

The direct and indirect effects of Alternative 2 will not measurably affect (change) the current 

landslide risk ratio of any of the watersheds (Table 6). The silvicultural prescriptions are mainly 

thinning from below in plantations which will have little effect on the root support in the 
Geology Report Craggy Vegetation Management Project

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watershed. The logging systems are a combination of tractor logging and skyline and have been 

designed either with project design features or project design components (i.e. silvicultural 

prescriptions, location of new landings, fuels treatment placement, etc.) to minimize effects to 

landslide processes.

Chatwin (1994) found scheduling activities with the use of wet weather operation standards and 

normal operating seasons was effective at reducing negative effects to hillslope hydrology 

especially on the occurrence of rilling and gullying, which will indirectly minimize negative 

effects to landslide risk. Cristan et al. (2016) concluded that roads, skid trails and stream 

crossings have the highest impact on hillslope hydrology changes including rilling and gullying. 

These activities should be given the most attention while planning best management practices for 

timber harvest in order to be effective in reducing effects.

Madej et al. (2012) found that most sediment delivered to the streams in the Panther Creek basin 

(Northwestern California) was from landslides originating on landings and roadbeds. This rate 

was reduced by improved road and landing construction standards, reduced ground disturbance 

from tractors for ground-based harvest and from treating legacy sediment sources. Litschert and 

MacDonald (2009) found that using best management practices meant only 19 of 200 sites that 

had undergone timber harvest had rills or gullies in the Sierra Nevada Forest Service lands. They 

attributed this low rate to the installation of effective waterbars on skid trails and the proper 

closing of skid trails before winter weather set in.

There are more than 20 project design features that are intended to reduce the probability of 

hillslope hydrology changes and changes to root support. The protection of riparian reserves 

including landslides, inner gorges and swales, in combination with partial harvest techniques that 

leave some understory vegetation intact have been found to substantially reduce the increase in 

landslide probability after timber harvest (Sidle 1992; Dhakal and Sidle 2003). Sidle (1992) 

found that partial cuts that retain the integrity of the understory vegetation maintain enough root 

strength to balance stand health needs and slope stability concerns. Cristan et al. (2016) 

concluded that implementation of best management practices was most effective in reducing 

effects to watershed processes when implemented both during treatment activities and at the 

close of the project.

There are project design features and design components that minimize the effects to root 

strength. The project design components include the silvicultural prescriptions and the prescribed 

fire prescriptions. The silvicultural prescriptions are essentially thinning from below and the 

prescribed fire is intended to have low to moderate burn severity in a mosaic pattern. These

silvicultural and burning prescriptions will have an extremely high effectiveness of maintaining 

sufficient root support across the watershed. The overall effectiveness of project design to 

maintain sufficient root strength for slope stability is extremely high (See Forest Plan Monitoring 

Reports on the Forest 

website: http://www.fs.usda.gov/main/klamath/landmanagement/planning).
Geology Report Craggy Vegetation Management Project

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Table 4: Summary of GEO Model Outputs and Percent of Watershed with High or Moderate Severity 

Disturbance under Alternative 2 for Indirect Effects and Cumulative Effects

7th Field Watershed Name Alt. 2 Risk Ratio 

(Indirect Effects)

Alt. 2 Watershed 

Disturbance 

Indirect Effects 

(% Watershed)

Alt. 2 

Cumulative 

Risk Ratio

Alt. 2 Watershed 

Disturbance 

Cumulative Effects 

(% Watershed)

Upper Humbug Creek 1.29 0% 1.29 0%

Middle Fork Humbug Creek 0.89 0% 0.89 0%

Lower Humbug Creek 0.47 0% 0.48 1%

Clear Creek 0.62 0% 0.62 0%

Greenhorn Creek 0.49 9% 0.49 9%

Yreka City-Yreka Creek 0.29 0% 0.29 0%

Long Gulch 0.59 0% 0.59 0%

Rocky Gulch-Yreka Creek 0.75 0% 0.75 0%

If a wildfire occurs in the project area after Alternative 2 treatments, the negligible negative 

effects in some of the 7th field watersheds (some don’t have a measurable effect) will be easily 

outweighed by the benefits to the landslide processes, by reducing the greater negative effects 

from a wildfire event. The assumption is that in any place where treatment occurs, the flame 

lengths will be less than four feet until about 10 to 15 years after treatment (see the fire and fuels 

report).

The assumption used to get results from the fire effects model is that a watershed is expected to 

have 25 percent of its area burn with moderate or high severity in a wildfire event under no 

action and under Alternative 2 about nine percent of the watershed will burn with high or 

moderate severity under Alternative 2. This is a reduction of the original percent expected to 

have high or moderate soil burn severity relative to the percent of the watershed treated.

With these assumptions that are dependent on a wildfire event occurring in the project area,

Alternative 2 is expected to have overall less negative effects on landslide processes than 

Alternative 1. As far as the analysis indicators of this analysis are concerned, Alternative 2 will 

reduce the landslide likelihood for Upper, Middle and Lower Humbug Creeks and Clear Creek

from almost certain to highly likely. Greenhorn and Long Gulch will both go from highly likely

down to likely. Yreka City-Yreka Creek will go from likely to unlikely. Rocky Gulch-Yreka 

Creek will stay the same as described under Alternative 1. For Alternative 2, Middle Fork and 

Lower Humbug Creek will move to a high landslide risk instead of going to a very high risk as it 

would under Alternative 1 if a wildfire event takes place. Greenhorn Creek will move to a very 

low risk; the landslide risks for the other watersheds remain the same as in Alternative 1 with a 

wildfire.

However, the risk ratios with Alternative 2 treatments and then a wildfire event are much lower 

than is described for Alternative 1 with a wildfire (see Table 6). When you consider the 

reduction in risk ratio between Alternative 1 and Alternative 2 under the wildfire scenario 

compared to the small increase in risk ratio (less than 0.01) that is a result of implementing 

Alternative 2 with a wildfire event, the benefits are clear. See Table 7 for results that compare

Alternative 1 to Alternatives 2 and 3 for burn severity percentages.
Geology Report Craggy Vegetation Management Project

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Cumulative Effects

The only action considered a reasonably foreseeable event that will have an effect on landslide 

risk is the same timber harvest plan mentioned in Alternative 1 and it will have the same effects 

on Alternative 2. The 0.01 increase to the risk ratio is not enough to increase the landslide risk 

for the watershed. This means that the cumulative landslide risk remains the same for all 

watersheds in the analysis area as described in the direct and indirect effects section above.

Alternative 3

Direct and Indirect Effects

The effects on landslide risk of Alternative 3 are the same as for Alternative 2.

Table 5: Summary of GEO Model Outputs and Percent of Watershed with High or Moderate Severity 

Disturbance under Alternative 3 for Indirect Effects and Cumulative Effects

7th Field Watershed Name

Alt. 3 Risk 

Ratio (Indirect 

Effects)

Alt. 3 Watershed 

Disturbance 

Indirect Effects 

(% watershed)

Alt. 3 

Cumulative 

Risk Ratio

Alt. 3 Watershed 

Disturbance 

Cumulative 

Effects 

(%Watershed)

Upper Humbug Creek 1.28 0% 1.28 0%

Middle Fork Humbug Creek 0.89 0% 0.89 0%

Lower Humbug Creek 0.47 0% 0.48 1%

Clear Creek 0.62 0% 0.62 0%

Greenhorn Creek 0.49 9% 0.49 9%

Yreka City-Yreka Creek 0.29 0% 0.29 0%

Long Gulch 0.59 0% 0.59 0%

Rocky Gulch-Yreka Creek 0.75 0% 0.75 0%

Cumulative Effects

The cumulative effects are the same as described in Alternative 2.

Comparison of Alternatives

Implementation of the action alternatives do not measurably change the landslide risk in a 

negative manner. The increase in risk ratios from the implementation of either Alternative 2 or 3, 

compared to Alternative 1, result in no more than a 0.01 increase. On average there is a 

substantially increased landslide risk if no action is taken and a wildfire event occurs compared 

to the risk if the action alternatives are implemented and then a wildfire event takes place.
Geology Report Craggy Vegetation Management Project

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Table 6: Summary of GEO Results for 7th Field Watersheds under Alternative 1 with a Modeled Wildfire 

Event and Action Alternatives with a Modeled Wildfire Event for Zero to Five Years after Project 

Implementation

7th Field Watershed Name Existing 

Risk Ratio

Alt. 2 or 3 

Risk Ratios

(Indirect 

Effects)

Alt. 1 Risk 

Ratio with 

Wildfire

Alt. 2 or 3

Risk Ratio

with Wildfire

Reduction 

in Risk 

Ratio from 

Fire Effects 

Under Alt.2 

or 3

Upper Humbug Creek 1.37 1.28 4.23 1.97 53%

Middle Fork Humbug Creek 1.10 0.89 2.48 1.54 38%

Lower Humbug Creek 0.52 0.47 1.82 1.09 40%

Clear Creek 0.61 0.62 1.86 1.10 41%

Greenhorn Creek 0.49 0.49 1.12 0.53 52%

Yreka City-Yreka Creek 0.29 0.29 0.77 0.25 68%

Long Gulch 0.59 0.59 2.16 1.38 36%

Rocky Gulch-Yreka Creek 0.75 0.75 2.94 2.06 30%

Table 7: Percent of Watershed that is Likely to Burn with High or Moderate Soil Burn Severity for all 

Alternatives

7th Field Watershed Name

Alt. 1 Percent of Watershed 

Likely to Burn With High or 

Moderate Soil Burn Severity

Alt. 2 or 3 Percent of 

Watershed Likely to Burn 

With High or Moderate Soil 

Burn Severity

Upper Humbug Creek 25% 9%

Middle Fork Humbug Creek 25% 18%

Lower Humbug Creek 25% 20%

Clear Creek 25% 16%

Greenhorn Creek 10% 9%

Yreka City-Yreka Creek 1% 1%

Long Gulch 10% 9%

Rocky Gulch-Yreka Creek 10% 9%

Table 8: Summary of Effects to Landslide Risk with and without Wildfire Scenario

Indicator Alternative 1 Alternative 2 Alternative 3

Landslide 

Risk with no 

wildfire 

scenario

There are three 7th field watersheds 

with high landslide risk (Upper, Middle 

Fork, and Lower Humbug Creek), two

with a moderate risk (Clear Creek and 

Rocky Gulch-Yreka Creek), two with a 

low risk (Yreka City-Yreka Creek and 

Long Gulch), and one with a very low 

risk (Greenhorn Creek).

Same as Alternative 1. Same as Alternative 1.
Geology Report Craggy Vegetation Management Project

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Indicator Alternative 1 Alternative 2 Alternative 3

Landslide 

Risk with 

wildfire 

scenario

There are two 7th field watersheds with 

a very high landslide risk Middle Fork 

and Lower Humbug Creek), three with 

high risk (Upper Humbug Creek, Clear 

Creek and Rocky Gulch-Yreka Creek), 

and three with low risk (Greenhorn 

Creek, Yreka City-Yreka Creek, and 

Long Gulch).

There are five 7th field

watersheds with high 

landslide risk (Upper, Middle 

Fork, and Lower Humbug 

Creek, Clear Creek and 

Rocky Gulch-Yreka Creek), 

two with a low risk (Yreka 

City-Yreka Creek and Long 

Gulch), and one with a very 

low risk (Greenhorn Creek).

Same as Alternative 2.

Compliance with law, regulation, policy, and the Forest Plan

Existing mapping was field verified by the Forest Geologist and unstable lands were removed 

from treatment areas where slope stability was not benefited. The landsliding magnitude and risk 

were analyzed for all of the alternatives in the project. The risk associated with the direct and 

indirect and cumulative effects of the project have been minimized by project design features and 

best management practices. The project is consistent with Forest Plan standards and guidelines as 

described in the Forest Plan Consistency Checklist in the project record, which is available on 

the project website.
Geology Report Craggy Vegetation Management Project

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