Figure 4-2. Stability and erosional sensitivity of South Fork Trinity River watershed (from DWR, 1992). NOT AVAILABLE IN ELECTRONIC FORMAT
Eastside watersheds, including Hayfork Creek
Because of their relatively low erosion rates and sediment yields, most watersheds draining the Hayfork and Rattlesnake Creek terrains fall into a moderate stability and erosion hazard category (CDWR, 1979). The more stable basins are generally underlain by more resistant bedrock types.
The Hayfork drainage is in relatively good condition. Slope stability adjacent to the main channel is good and significant channel aggradation did not occur following the 1964 flood (Irizarry, et. al., 1985). Rather, current and lingering impacts are related to local placer mining, reduction in riparian vegetation (leading to thermal problems), leaching of contaminated water from private septic systems and industrial areas, bank destabilization from grazing impacts, loss of riparian forests, and road-generated sediment (Irizarry, et. al., 1985). Gravel removal may also have locally accelerated channel degradation and the loss of spawning gravels and other habitat types (Stokely, personal communication, from Yount, personal communication). Water drafting and poor riparian conditions in Hayfork Valley have reduced summer flows and contributed substantially to high summer water temperatures (Haskins and Irizarry, 1988).
There are no widespread areas of sensitive lands in the Hayfork watershed, but local areas are steep, prone to extreme surface erosion or contain slope instabilities. For example, surface erosion in Salt Creek, lower Hayfork Creek and in Corral Creek has lead to some infilling of pools and channel impaction in the lower mainstem (Haskins and Irizarry, 1988). Portions of Rusch Creek, Bear Creek, Miners Creek, Corral Creek, Tule Creek and Little Creek basins are all underlain by erodible dioritic soils which are very sensitive to management impacts.
Mass wasting has locally caused sediment problems in these eastern tributaries, but active landsliding is not widespread. For example, Rattlesnake Creek terrane is characterized by abundant inactive and ancient landslides covering 13% of the terrane, but only 1% of the terrane is underlain by active instabilities (Table 4-3; CDWR, 1979). Rattlesnake Creek terrane contains many of these locally active debris slides and earthflows. Thus, upper West Tule Creek has been judged too unstable for timber management (John Veevaert, personal communication). Although watersheds underlain by steeper portions of Rattlesnake Creek Terrain are locally unstable and generate significant turbidity, other sub-basins in the same rock type have been characterized as "resistant" to timber management (CDWR, 1979), primarily due to gentler topographic expression.
Other basins which are not inherently unstable, have experienced significant erosion and sedimentation problems due largely to intensive land use activities. Major eastern tributaries which drain directly to the South Fork Trinity River, such as the East Fork of the South Fork, Rattlesnake Creek and Butter Creek, have significant problems related to cumulative effects of intensive timber management over wide portions of the watersheds (Irizarry, et. al., 1985). It has been recommended that these watershed areas be managed with greater emphasis on preventing cumulative effects. Special management practices should be employed and proposed levels of harvest should be modified (lowered) in some basins.
Past placer mining in upper Hayfork Creek and the East Fork of Hayfork Creek has contributed to channel instability, broad, shallow channels, a lack of pools and poorly established riparian vegetation. Grazing has contributed to channel instability in Hayfork Valley, Salt Creek, Carr Creek and Barker Creek, and water drafting in Tule, Salt, Big and Hayfork Creeks has contributed to seasonal low flows and warming. The Hayfork Creek watershed is perhaps more sensitive to grazing, gravel removal and mining practices, which directly cause channel impacts, or to agricultural and domestic water drawing practices, which elevate summer steam temperatures, than to an overabundance of coarse streambed sediment (Irizarry, et. al., 1985). Some recovery of channel stability and riparian vegetation may have occurred in Barker Creek where the range was closed to grazing in 1985 (Stokely, personal communication).
Overall, the Hayfork Creek watershed can be characterized as being moderately sensitive to cumulative erosion and sedimentation effects (Haskins and Irizarry, 1988). Certain east-side drainage basins, including Butter Creek, Rattlesnake Creek, East Fork South Fork, and upper Hayfork Creek, among others, are considered to be approaching their individual Thresholds of Concern (TOC) for cumulative watershed effects (Haskins and Irizarry, 1988).
Westside watersheds draining South Fork Mountain
High sediment production from other tributary basins, and high turbidity in the main stem South Fork Trinity River, has been linked to the combined action of human activities and natural processes. Soil loss was especially severe in unstable geologic terrain and on fragile soils where high sediment yields were generated during high intensity storms (CDWR, 1979). High turbidities were often associated with timber harvests and road construction, and are related to several factors including slope, aspect, geology, soils, rainfall and method and quality of logging operation (CDWR, 1979).
The California Department of Water Resources (1979) traced excessive turbidity back to tributary basins largely draining the South Fork Mountain schist, Galice Formation, Franciscan complex sedimentary rocks and steeper portions of the Rattlesnake Creek terrane. The most turbid tributaries were found to occur between Forest Glen and the confluence with the main stem Trinity River, with Grouse Creek being the most turbid. These western tributaries exhibit significant instability, especially toward the north end of the South Fork Trinity River basin (Irizarry, et. al., 1985).
Active debris slides are most common along oversteepened slopes of the inner gorge of the main stem South Fork Trinity River, and Grouse, Madden and Pelletreau Creeks. Gutted streams, scoured by debris torrents, are most common on the eastern slope of South Fork Mountain, south-southwest of Hyampom, and in areas logged before the 1964 flood (CDWR, 1979). West side tributaries, including Cold Springs, Johnson, Hitchcock and Pelletreau Creeks, were the most severely damaged.
Kojan (1974; 1976) defined three landslide prone zones on South Fork Mountain. These are 1) the inner gorge of the South Fork Trinity River, 2) the inner gorge of secondary tributary streams, and 3) interfluve divide areas. He recommended a 1.0 km-wide zone adjacent the South Fork Trinity River be excluded from logging and vegetation disturbance because of potential debris slides and deeper translational landsliding. For the same reason, he suggested the narrower inner gorges of tributaries also be left undisturbed.
The erosionally sensitive and unstable geologic terranes underlying South Fork Mountain, especially those areas downstream from Forest Glen, have shown the greatest erosional impact resulting from land use and floods. During the 1964 storm, many tributary streams on South Fork Mountain were gutted by debris torrents and streamside landsliding, especially on and downstream from heavily logged lands. LaFaunce (1975) claimed that continued logging on sensitive terrains caused erosion and sedimentation for years after the flood, and this impact is thought to continue today (Haskins and Irizarry, 1988).
MacCleery (1974, p.141), in his study of 13 small tributaries in the highly erodible and unstable South Fork Mountain area, reported that "...South Fork mountain is not an ambiguous laboratory. It is highly consistent from end to end in slope, aspect, topography, geology and rainfall patterns. The only way in which one part of the mountain differs sharply from another is the extent to which one part may have been extensively logged and another part remains forested. That the logged areas suffer heavy damage during and after storms and the unlogged areas do not, is clear."
In the past, generally similar landuse techniques and practices were broadly applied to all geologic and landform types across the basin with little respect to terrain sensitivity. As a direct consequence of the inherent differences in stability and erodibility of South Fork geologic terranes, landuse effects have been found to vary dramatically from basin to basin (Haskins and Irizarry, 1988)(Plate 2).
In recognition of these differences, land management on USFS lands during the last 15 years have been tailored to local conditions and significantly altered in acknowledgement of inherent differences in erodibility and geologic stability of different sub-basins. Areas have been set aside as being incompatible with harvesting and routine timber management activities because of their instability.
Private lands timber management is subject to regulation by State Forest Practices Act and associated Forest Practice Rules. The first version of these rules and standards were implemented in the early 1970's, after much of the South Fork mountain private timberlands had already been harvested.
Impacts from cutover private timberlands have been severe. Heavily logged and roaded watersheds have yielded millions of cubic yards of sediment to the South Fork Trinity River, filling pools and destroying riparian vegetation (Irizarry, et. al., 1985). Twenty years later, millions of yards of sediment still reside in the main channel and its tributaries as channel fills and terrace deposits, creating a wide floodplain, shallow riffles and pools. The in-stream sediment sources alone will provide the main stem South Fork Trinity River with large quantities of bedload sediment for decades to come.
Forestry regulations and private land forest practices have improved substantially over the last 20 years, but the legacy of abandoned roads, failing crossings and unstable slopes remains as perhaps the most serious threat to the continued existence and recovery of depleted downstream fisheries resources. Recovery of fish diversity and population will be directly dependent on the identification and treatment of potential sediment sources on older cut-over lands, as well as the future application of state-of-the-art land management practices and improvement work on both National Forest System and private holdings (Irizarry, et. al., 1985).
Earth Science Associates (1980) outlined those factors believed most likely to have adversely affected fisheries in the lower Klamath River tributaries. In descending order of importance these include : 1) logging, 2) the 1964 flood, 3) the 1976 drought, and 4) over fishing. Indeed, the effects of the 1964 flood and erosion coming from poorly managed lands in the western and central portion of the South Fork Trinity River basin led to destruction of valuable spawning, rearing and holding habitat, and resulted in the long term reduction in the numbers of anadromous fish (Haskins and Irizarry, 1988). To date the South Fork Trinity River has not recovered from this event.
The physical and biological effects of erosion and subsequent sediment yield from logged lands during the 1964 flood have been severe and long lived. This flood, more than an other single event, has been identified as a triggering mechanism that resulted in the dramatic loss of fish habitat and fish populations in tributaries and the lower main stem of the South Fork Trinity River. Cumulative watershed effects, including channel aggradation, widening, bank undercutting and general channel instability resulted during the 1964 flood (Haskins and Irizarry, 1988; USFS, 1990I).
Fisheries have not recovered from these effects, even though some tributaries in relatively stable watersheds have regained much of their original channel characteristics. Many lower South Fork Trinity River tributaries, and the lower main stem of the river, are still heavily aggraded and continue to display highly mobile beds and impacted riparian zones (Plate 2).
Existing land conditions on South Fork mountain, especially privately managed lands, have been identified as posing a serious threat to the recovery of anadromous fisheries in the South Fork Trinity River. Many of the same "sensitive" watersheds on South Fork mountain terrain which "fell apart" during the flood have again been primed by continued landuse (harvesting, road construction and road abandonment) to deliver large quantities of sediment to downstream areas during the next major flood event. Regardless of the flood's magnitude, there is a direct link between past logging and severe watershed damage (MacCleery, 1974). Areas that were not heavily logged and roaded did not show comparable damage.
Debris torrenting, streamside landsliding, and stream "gutting" are likely to again occur in many of the these basins with a similar sized storm. If a significant flood occurs in the next several decades, portions of recently burned watersheds in the upper South Fork Trinity River are also likely to experience substantial erosion and widespread mass slope failure. Degradation will continue as roots decompose and accelerated mass wasting occurs, further contributing to cumulative watershed effects (USFS, 1990i; see also Fire Effects, this chapter).
Prior to the flood, the South Fork Trinity River was characterized by "scattered, large, deep pools interspersed with shallow pools, riffles and rapids." (USFS, 1990i). Immediately after the flood, most large pools had been infilled and low gradient sections of the channel had aggraded and widened. Today, the middle South Fork Trinity River is wide, with shallow pools and riffles and a laterally migrating thalweg while lower reaches in the bedrock gorge alternate from pools to aggraded sections of channel (USFS, 1990i). Aggradation extends from the mouth of Sulphur Glade Creek nearly continuously downstream to the mouth near Salyer.
Hillslope erosion triggered by the 1964 flood resulted in substantial changes in the streambed of the main South Fork Trinity River. Channel changes were most dramatic in the lower basin, from Hyampom downstream, where aggradation approached 30 feet in some locations. Locations of aggradation during flood events were largely controlled by localized inputs of sediment from high yield tributary watersheds, such as Pelletreau Creek and Grouse Creek, and severe aggradation is found at locations with diminished channel gradients and increased channel widths, such as at Hyampom Valley.
Hickey (1969, in SCS, 1972) reported streambed elevation increases at USGS gaging stations along the South Fork Trinity River, from 1964 to 1965, as follows:
South Fork near Salyer: +12.9 feet
South Fork at Forest Glen: + 2.2 feet
Hayfork Creek near Hyampom: + 0.6 feet
Hayfork Creek near Hayfork: + 0.3 feet
Documented channel bed changes following drainage basin erosion in 1964 are still serving to limit the recovery of the salmonid fishery, especially in the main stem. Channel stored sediment delivered to the drainage system 20 years ago is continually remobilized and moves down the channel system during storm events. Millions of cubic yards of sediment remain in the lower reaches of the tributaries as terraces and channel fills (Haskins, 1981). Persistent physical effects include filling of pools and deposition of fine sediments in areas which were once important to main stem spawning and rearing.
Channel fill deposits in the main channel of the upper South Fork Trinity River, above Forest Glen, have largely been removed and the profile has recovered its pre-1964 profile (Cruz, 1992, personal communication) Terrace remnants from the 1964 flood are locally preserved to show the extent of past channel filling, and these deposits provide a small but persistent source of sediment to downstream areas. Some pools remain partially filled with fine sediment. However, landsliding and erosion from lands burned during recent wildfires threatens to increase channel sedimentation.
In some sections of the upper South Fork Trinity River channel, where gradients were relatively high or the channel is naturally constricted, deep channel filling apparently never occurred. For example, at the Forest Glen gaging station 1965 thalweg elevations were only 2.2 feet above 1964 (pre-flood) elevations.
In contrast, Hyampom Valley contains huge quantities of recently introduced sediment which is in intermediate and long term storage. Much of this sediment came from high yield tributary streams entering along the western margin of the valley. This material is stored as a valley fill deposit to measured depths of over 25 feet (CDWR, 1982). USGS records show that the thalweg of the main South Fork Trinity River channel at the stream gaging station near the upper end of the valley retained at least 1.5 meters of aggradation by 1980, but the channel had widened by more than 20 meters. Approximately 40% of the Quaternary alluvium, which occurred as elevated terrace deposits in the valley before 1964, was eroded and replaced with a fresh layer of flood gravels from 1964 flows (CDWR, 1982).
Channel width increased, and persisted, in most locations where flood gravels were deposited in 1964, and this widening has impacted and continues to affect the ability of some reaches of the South Fork Trinity River to support healthy anadromous fisheries. Filled pools and widened channels have resulted in comparatively shallow summer low flow conditions and increased water temperatures. The main channel, from Hyampom Valley downstream, experienced the greatest amount of landsliding and channel widening. Channel segments which still retain flood deposits (eg, South Fork Trinity River below Forest Glen) are subject to thalweg meandering, active channel migration and increased flooding (CDWR, 1982). The streambed in these locations is dynamic and highly mobile.
Channel aggradation has not only resulted in widening, shallowing, and loss of in-channel structure, in many cases mature riparian zones have been destroyed and lost for decades (Haskins and Irizarry, 1988). Constantly shifting thalwegs and unstable gravel substrates in the main channel and in the most severely aggraded tributaries have prevented the reestablishment of riparian vegetation. Rapid downstream increases in main stem water temperature below the wilderness is thought to be partially caused by loss of riparian vegetation and channel widening (Cruz, personal communication; Ranken, personal communication).
The absence of significant riparian canopy in Hyampom Valley may also be affecting stream water temperatures (Haskins and Irizarry, 1988). These factors together - widening, aggradation, shallowing, mobile bed materials and loss of riparian vegetation - have severely diminished the ability of the main channel to provide adequate spawning and rearing habitat for summer steelhead and spring chinook (Plate 2). Until excess sediment is transported through the South Fork Trinity River, and out of the valley, conditions are not likely to improve. According to Haskins and Irizarry (1988), the Hyampom section of the South Fork Trinity River is considered highly sensitive to continued cumulative watershed effects. Chapter 4 continued