Erosion and sedimentation has been identified by a number of watershed scientists and fisheries experts as a limiting factor which initiated the precipitous decline of salmonid populations in the South Fork Trinity River (CDWR, 1979; 1982; Haskins and Irizarry, 1988). It has also been identified as a significant factor limiting fisheries recovery in the basin (CDWR, 1982; Irizarry, et. al., 1985)
The South Fork Trinity River has been identified as one of the highest sediment producing streams in the Klamath and Trinity River system (SCS, 1972). The mean annual sediment discharge for the South Fork Trinity River, from 1940-1965, is reported at 1,650 tons/mi2/yr (SCS, 1972). The U.S Geological Survey reports average annual suspended sediment discharge for the South Fork Trinity River from 1958 to 1964 at 734 tons/mi2 (Hawley and Jones, 1969). They estimate that bedload discharge is six percent of average annual suspended sediment discharge. The much lower USGS yield probably reflects the occurrence of smaller storms which occurred during the shorter measurement period. Estimates of annual sediment yield based on longer measurement periods are generally more accurate and better reflect long term sediment transport rates.
Published annual suspended sediment discharge rates for the South Fork Trinity River are lower than those for the coastal watersheds of the Eel (4,300 t/mi2), Van Duzen (6,900 t/mi2) and Mad Rivers (2,900 t/mi2) (Hawley and Jones, 1969). These watersheds are underlain by erodible and unstable Franciscan assemblage rocks. Basins underlain by more resistant bedrock types of the Klamath Mountain Province, such as the Trinity River at Lewiston (160 t/mi2) and the North Fork Trinity River at Helena (210 t/mi2) display noticeably lower suspended sediment yields than the South Fork Trinity River (Hawley and Jones, 1969). Only the western portion of the South Fork Trinity River basin is underlain by the unstable and highly erodible Coast Range lithologies, and most sediment discharged from the watershed is thought to originate from this terrain (CDWR, 1992).
Sediment transport modeling, based on two sediment transport equations, were run for three gaging station sites along the South Fork Trinity River (CDWR, 1992). The calculated yields were based on surveyed cross sections of the main channel completed in 1990. This data suggests total sediment yields of 4,200 tons/mi2/year would be expected if measured flows from 1961-1990 were run through the 1990 channel cross sections (Table 4-1). According to the calculations, extremely high sediment yields would be expected from the lower South Fork Trinity River basin. Lack of historical cross section data, especially before and after major flood events, makes this calculated value more an estimate of potential sediment yield, given current channel conditions, than actual past yield.
Table 4-1.Estimated total sediment yield, 1961-1990 (modified from CDWR, 1992)
Total Sediment Cumulative Sediment
Area Load Yield
Sub-basin
yd3/mi2 tons/mi2per
(mi2) yd3x103 tonsx103 per yr. yr.
Headwaters to
Forest Glen 208 5,832 8,660 930 1,388
Forest Glen to
Hyampom + Hayfork 556 10,261 15,238 620 914
Creek
Hyampom downstream
to Salyer 134 60,148 89,320 15,000 22,219
Entire South Fork
Trinity River 8982 76,241 113,218 2,830 4,203
Basin3
1 Assumes density of 110 lbs/ft3
2 Drainage area upstream from USGS gaging station
3 Above Salyer gaging station
Within the South Fork Trinity River basin, total bedload input and transport is thought to increase in a fairly linear rate downstream, through three sampling stations at Forest Glen, Hyampom and Salyer (CDWR, 1992, p. 5). While a large amount of sediment is introduced between Forest Glen and Hyampom (largely from South Fork Mountain tributaries), 4 to 5 times that amount is introduced between Hyampom and Salyer (Table 4-1; CDWR, 1992).
Viewing the data as sediment yield per unit area (tons/mi2/year) provides great insight into estimated sediment yield from various parts of the watershed. Cumulative sediment yield from the area below Hyampom suggests this region discharges 24 times the unit sediment yield rate of the middle third of the basin, between Forest Glen and Hyampom (including the large Hayfork Creek sub-watershed)(Table 4-1). Certainly, a large proportion of the sediment delivered to the main channel of the South Fork Trinity River has come from west-side tributaries downstream of Hyampom (eg., Raines and Kelsey, 1991), while there has been a relatively low unit input from the Hayfork Creek sub-drainage (Haskins and Irizarry, 1988).
Extensive unstable areas are present within the South Fork Trinity River watershed. These include landslides, unstable inner gorges and dormant features which are highly susceptible to activation by natural events (large floods) or management-related land disturbances (road building and timber harvesting). Two significant sources of sediment have been identified as adding to the sediment load of the stream each year. These include active landslides and remobilized channel flood deposits (stream bed and bank erosion) (Irizarry, et. al., 1985). An estimate of the magnitude of the various sources of sediment yield in the basin, between 1940 and 1965, suggests that landsliding and stream bank erosion account for over 80% of the basin's yield (Table 4-2; SCS, 1972).
Table 4-2. Estimated sediment sources, South Fork Trinity River basin (SCS, 1972).
Sediment yield
Sediment source
(acre-ft tons per Percent of total
per year) mi2/year1 yield
Landslides 570 1,521 50
Streambanks 380 1,014 33
Sheet/Gully 190 507 17
Total 1,140 3,042 100
1 Assumes 110 lbs/ft3 density and 898 mi2 drainage area
Raines and Kelsey (1991), in a much more recent and detailed sediment source inventory of the Grouse Creek basin, documented rates of sediment production of 4,130 tons/mi2/yr, the highest of published rates for disturbed, forested watersheds in the Pacific Northwest. Sediment production was concentrated during major storm periods, in proximity to roaded areas, and in zones of geologic instability. These were the dominant controls on rates of sediment production in this large tributary watershed. According to the authors, over 85% of the basin's sediment production was generated by streamside landsliding, with the remainder coming from hillslope erosion, road-related erosion and streambank erosion.
The South Fork Trinity River contains a variety of geologic terrains and soil types. As a result, sub-watersheds and major tributaries display widely variable levels of erosional sensitivity, natural stability and resistance to land use impacts and disturbances (CDWR, 1979). The basin in underlain by nine (9) distinct geologic types, each of which displays differing levels of natural stability and erosional sensitivity (Table 4-3). These geologic units are grouped into two major terranes or "belts": the Western Paleozoic and Triassic Belt and the Western Jurassic Belt (Figure 4-1).
Three parallel subunits, or geologic terranes, divide the rocks of the Western Paleozoic and Triassic Belt. These include:
Rattlesnake Creek terrane: This group contains a heterogenous mixture of rock types separated by shear zones. The terrain is noted for its instability.
Hayfork terrane: The Hayfork geologic terrane contains an eastern and a western belt of rocks. The western belt is composed of fairly resistant rocks slightly prone to surface erosion and some debris sliding. The eastern belt is composed of a melange (mangled rock), like the Rattlesnake Creek terrane, and is highly unstable locally. The Ironside Mountain batholith, in the southern area, is noted for surface erosion problems characterized by weathered, detachable granular soils and has local areas of debris slide potential.
North Fork terrane: This geologic group, similar to the Rattlesnake Creek terrane, is a melange but is exposed in only a very small portion of the eastern watershed of Hayfork Creek.
Figure 4-1. Geologic terranes in the South Fork Trinity River basin (from CDWR, 1979).
NOT AVAILABLE IN ELECTRONIC FORMAT
Three parallel subunits, or geologic terranes, also divide the rocks of the Western Jurassic Belt of rocks (Haskins and Irizarry, 1988).
These include:
Galice Formation: The Galice, located in the western portion of the basin, is noted for debris slides and instability along shear zones within low grade metamorphic rocks, and within the inner gorge of the South Fork Trinity River.
Weaverville Formation: These young rocks of the Klamath Mountain province are found in Hyampom and Hayfork Valleys and are composed of poorly consolidated Eocene non-marine sediments.
Franciscan Assemblage: This group of rock types is composed of 3 belts; Coastal, Central and Yolla Bolly, and includes the South Fork Mountain schist and sandstones and shales. This schist is noted for its instability and the rock types are prone to instability and gullying elsewhere.
Table 4-3. Landslide occurrence in geologic units of the South Fork Trinity River basin
(CDWR, 1979).
Area Covered
Geologic Unit
Percent of Unit in
Total Area Active, inactive and ancient
landslides
(mi2) (%) Active Ancient Total
Franciscan
Assemblage 62.7 (7.1) 3 8 11
South Fork Mountain
schist 122.2 (13.8) 3 12 15
Galice Formation
101.3 (11.4) 6 7 13
Rattlesnake Creek
Terrane 278.8 (31.4) 1 13 14
Hayfork Terrane 177.8 (20.0) 0.1 1.3 1.4
Plutonic Rocks 122.5 (13.8) 0.4 1.5 1.9
Weaverville
Formation 20.1 (2.3) 0.2 1 1.2
North Fork Terrane
0.4 (0.05) 0 20 20
Rocks of Great
Valley Sequence 1.5 (0.1) 0 1.7 1.7
Total 887 (100%) --- --- ---
The Galice formation has the largest percent of active landslides per unit area (6%), nearly twice the South Fork Mountain schist (3%), largely because it underlies the steep inner gorge slopes along the lower South Fork Trinity River (Table 4-3). However, the South Fork Mountain schist covers a larger area of the basin. Nearly 90% of the 40 mile long eastern slope of South Fork Mountain, totalling 100 mi2, is underlain by South Fork Mountain schist (Figure 4-1)(Haskins et al., 1980).
South Fork Mountain schist has strongly developed structure (foliation), and the deep fine grained soils have lead to widespread past and recently active landsliding (Haskins, 1981). The east dipping foliation is a natural detachment surface for rotational-translational landslides and block-glide landslides. Nearly the entire eastern slope of the mountain is covered by nested dormant rotational-translational slides (Haskins et al., 1980). Due to the dense vegetation, many of the landslides on South Fork Mountain are not easily recognized on aerial photos. Those on inner gorge slopes are most easily seen.
Active landslide frequency in the Galice terrain is sixty times higher than in the comparatively stable Hayfork terrane (0.1%), and 15 times higher than in the hard plutonic rocks that underlie western portions of the Hayfork Creek basin. The Rattlesnake Creek terrane is characterized by abundant inactive and ancient landslides (13% of the terrane), but active landsliding is far less common here also. Mapped, inactive landslides are most common on South Fork Mountain schist, followed closely by Rattlesnake Creek terrane and the Franciscan Assemblage (Table 4-3).
CDWR (1979, Plate 4) has assembled an instability and erosion hazard map for the South Fork Trinity River basin, showing the areas of moderate, high, very high and extreme hazard (Figure 4-2). The zones are described as follows:
Moderate: Generally stable slopes and geologic aspect; occurs mostly in metavolcanic rocks of Hayfork Terrane; generally lower precipitation than rest of basin; also includes valleys of Hayfork and Hyampom.
High: Moderately unstable slopes or erodible soils; abundant inactive and/ or ancient landsliding; less abundant active slides; road cut and fill failures, and moderate rilling and gullying; sheared and broken metamorphic rocks of Rattlesnake Creek Terrane and landslide-prone parts of Hayfork Terrane.
Very High: Unstable geologic aspect and erodible soils; abundant active, inactive and ancient landslides; highly erodible sub-soils; stream gutting and gullying to be expected below harvest areas; includes unstable areas of Rattlesnake Creek Terrane, Galice Formation, Franciscan Assemblage and most of South Fork Mountain schist.
Extreme: Highly unstable and erodible soils; generally includes a combination of unstable geology, high rainfall, deep soil creep, stream undercutting, and/or steep slopes; occurs mostly in especially sensitive areas of South Fork Mountain.
It is apparent that bedrock geology, and the overlying soil, has a great influence on slope stability and erosional sensitivity of lands throughout the basin. This influence has been locally heightened, sometimes to a great degree, by the overprint of timber harvesting and road construction. These two factors have, in many locations, combined to cause deteoriated stream channel conditions (Plate 2) and cumulative watershed effects.
Perhaps the most important watershed characteristic that directly affects a watershed's denudation rate and total annual sediment yield is its land use history (SCS, 1972). This is especially evident in the South Fork Trinity River basin, where "the cumulative impact of roads, landslides, timber harvests, and wildfires, coupled with long intense storms, appears to be the primary cause of soil loss, watershed degradation, and [elevated] turbidity" (CDWR, 1992, p.16).
On the basis of water quality sampling in the late 1970s, CDWR (1979) found that most of the South Fork Trinity River basin does not contain geologically sensitive terrain, and human activities do not cause excessive stream turbidity throughout large portions of the basin. The resistant terrains are relatively insensitive to timber management, and show relatively small increases in turbidity. These stable basins are found mostly on the west-draining slopes of the South Fork Trinity River basin, including Hayfork Creek, and are underlain by geologic types of the Hayfork terrane and more stable parts of the Rattlesnake Creek terrane (Table 4-3).
Most of these low-to-moderate yield areas have not received detailed geologic analyses comparable to those conducted in the unstable, erodible, South Fork Mountain tributary basins. Aside from timber sale assessments, there is little quantitative data on sediment sources and sediment production rates for these sub-watersheds. Most stability, erosion hazard and sediment data is observational and has been collected during stream surveys (when channel and watershed conditions are generally assessed; see Chapter III), following wildfires (in preparation for fire rehabilitation work), or as a result of more recent road inventories and watershed assessments (see Chapter XI). Chapter 4 continued