Landslides, Freshwater Creek Watershed, California, 1999

The discussion below was excerpted from OFR 99-10 and 99-10a.

The geology of the drainage is dominated by the Freshwater Fault. This fault is
considered a normal fault with blocks on either side stepping down to the west in a
regional context. Multiple strands of the fault appear to be present in the study
area. The fault is considered inactive because no evidence of movement after late
Quaternary time has been observed along it. The Freshwater Fault splits the
watershed into two sections having different geologic conditions, topographic styles
and potentials for landsliding (Plate 1).

Much of the western half of the drainage has relatively steep topography underlain by
gently dipping, interbedded mudstone, siltstone, sandstone and minor conglomerate of
the Wildcat Group (early Pleistocene to late Miocene). These rocks are typically prone
to shallow debris flows/slides and occasional translational/rotational rock slides.
The Wildcat Group in this part of the drainage rests uncomformably atop interbedded
sandstone and shale of the Yager Formation (early Tertiary). The Yager Formation dips
steeply to the northwest (Plate 1). Only one translational/rotational rockslide was
identified in this unit. Several lineaments were noted on the aerial photographs
throughout this part of the drainage, but no faults are indicated.

The geology of the eastern half of the drainage is complex and most of the larger
translational/rotational landslides and earthflows are found there. The western third
of this area is underlain by Wildcat Group rocks that rest unconformably atop dense
sandstones and shales of the Yager Formation and Coastal Belt of the Franciscan
Assemblage (PLATE 1). The contacts between Yager and Franciscan rocks probably
represent multiple strands of the Freshwater Fault (Keith Knudsen, May 1999, personal
communication).

The remainder of the eastern half is dominated by a series of northeast-dipping
thrust faults. These faults are considered inactive at this time because they are
not indicated on the State of California Earthquake Fault Zone map (State of California,
1983). The thrust faults juxtapose irregular slabs of dense sandstone/shale and melange
of the Coastal Belt of the Franciscan Assemblage. The erosional contrast between the two
units is seen as discrete bands of "hard" and "soft" topography that cut through the
drainage in a north-westerly direction (Plate 1).

The characteristics and properties of the various rock units found in this area were
taken into account during the assessment of relative landslide potential. The primary
sources of geological data used in preparing the relative landslide potential map are
compiled on Plate 1 of Open-File Report 99-10.

The Relative Landslide Potential Map has been prepared to aid in resource management and
general land-use planning. It is not intended, nor should it be used for, evaluation of
specific sites. Site specific evaluations often require detailed engineering geologic
studies and soil engineering investigations of the underlying soil and bedrock for proper
planning of specific projects.

This map is derivative, based on the following: 1) the occurrence and distribution
landslides, other types of slope failure, and features indicating slope instability
(Plate 1, Open-File Report 99-10); 2) the geology of the area, including bedrock types
and lithologic properties relative to slope stability and distribution of various earth
materials, as well as the structural framework, such as the folded and faulted strata
found throught the region; and 3) the relative behavior of slopes within the area as
interpreted from analysis of historic aerial photographs and recent field observations.

Studies of the stability of specific sites commonly require development of quantitative
data through laboratory testing of field samples. This level of testing was not done for
this regional evaluation. In producing this map, it was assumed that actively sliding
material has the lowest relative strength, and thus, the highest relative potential for
landsliding of all the geological materials underlying the slopes. Recent alluvial
deposits in the valley bottom were assumed to possess the least potential for landsliding
due to their flat slope. The relative potential for landsliding between these extremes
was evaluated subjectively based on aerial photograph interpretation, field observations
and the following principal factors:

The broad apparent stability characteristics of geological materials underlying
the slopes and adjacent lower-lying areas, as expressed in their natural
exposures and their observed responses to alteration by land-use activities.
For example, slopes that exhibit abundant evidence of landsliding or downslope
creep of the soil are considered oversteepened relative to the strength of the
materials that underlie them.

Steepness of slopes, whether or not landslides are apparent.

The presence of active or intermittent natural influences that tend to cause
slope failure. These include gravity, climatic conditions, fluvial processes,
and the tendency of certain soils to shrink and swell under varying moisture
conditions.

These criteria are combined to yield the five-value scale used on the Relative Landslide
Potential Map to indicate the comparative capacity of the slopes within the map area to
resist failure by landsliding.

The map, Landslide Potential with Geologic and Geomorphic Features, Freshwater Creek,
Humboldt County, California is a digital representation of the map, Geological and
Geomorphic Features Related to Landsliding, OFR 99-10, combined with the map of Relative
Landslide Potential, OFR 99-10a.

Purpose:

The maps/data are useful in preparation of Timber Harvesting Plans, and for use by land
managers for identifying areas of potentially unstable ground. The maps/data serve as a
guide to potential problem areas where more site-specific review is required.

Data Capture Procedures:

As noted previously, geomorphic features were mapped on stereo pair photographs and
transferred to 7.5 minute quadrangle mylar overlays. These overlays were scanned and
converted to ArcInfo vector coverages.