APPENDIX E Interpretation of Current and Historical Sediment Movement from Bedrock Exposed in Hells Canyon

This page left blank intentionally. Interpretation of Current and Historical Sediment Movement from Bedrock Exposed in Hells Canyon This appendix provides a more detailed hillslope analysis for the Snake River within Hells Canyon. Specifically, a slope classification analysis is followed by a detailed discussion regarding rock varnish. The objective of this appendix is to provide a semi- quantitative evaluation of sediment sources to the local tributaries and mainstem from hillslope processes.

E.1. Slope Classifications Figure 5.15 in the main report shows the five slope classifications for hillside slopes in the Hells Canyon drainage basin (excluding the Imnaha River and Salmon River tributary drainage basins). Each of the five slope classes show the hillside surface area that is classified within each of these slope classes. Most of the surface area (52 percent) in the drainage basin has a hillside slope between 40 and 60 degrees (Photos 4 and 7) (Table 5.15). Further, an additional 10 percent of the surface area has an unusual slope greater than 60 degrees from horizontal. Much of the earth’s surface would be adequately described using hillside slopes within the first three slope classes of less than 10 degrees, 10 to 30 degrees, and 30 to 40 degrees). For example, floodplains frequently fall within the less than 10-degree slope class and hillsides adjacent to the floodplain are frequently within the 10- to 30-degree slope class. These slopes result because most of these surfaces have been exposed for sufficient geologic time to weather and erode accordingly. The dominantly high slopes in Hells Canyon indicate a very young surface; one that has been exposed to earth surface weathering processes and rapid downcutting by the Snake River for only a very short geologic time.

Steep slopes typically weather to a “maximum angle of slope at which loose, cohesionless material” of similar composition “will come to rest” (Gray et al. 1974) called the angle of repose. The angle of repose commonly ranges between 33 and 37 degrees from the horizontal and is rarely less than 30 or more than 39 degrees. Therefore, the third slope classification ranging between 30 and 40 degrees represents the angle of repose, where loose particles on most sediment mantled slopes cluster at incipient motion. In other words, within this slope class, sediment particles move tending to slide or otherwise easily move downslope and the slope is subsequently retained within this slope range. Therefore, most of the surface with 30- to 39-degree slopes is likely to be loose material of variable particle sizes forming scree slopes and talus piles, while surfaces exceeding this slope are almost certainly bedrock. In bedrock-dominated environments such as Hells Canyon, bedrock surfaces in the upper two slope classes will tend toward catastrophic episodes of failure generally in the form of a landslide as a result of natural earth surface weathering processes, particularly surfaces in the greater than 60-degree slope class. E.1.1. Slopes Less Than 10 Degrees There are several very unusual characteristics to the surface locations within slope classes of the Hells Canyon drainage. The lowest slope class, surfaces with less than 10 degrees, are usually found associated with the floodplain of a river and this is also largely true for the Snake River in the Hells Canyon and for the terraces created by the Bonneville Flood. However, most of the significant areas in this slope class are in the uppermost headwaters of the tributary creeks. With the exception of the alluvium and terrace gravels, this positioning is the inverse of the typical drainage where drainage basins increase in slope toward the headwaters and have their highest surface slopes in the headwater area where bedrock or more resistant alluvial sediments impede headcutting. The headwaters of Wolf, Divide, Cherry, and Deep creeks are the major creeks with greater than 5 percent of their total surface area in the less than 10-degree slope class (Table 5.6), with most of this surface area in the uppermost headwaters adjacent to Hells Canyon drainage divide rather than adjacent to the Snake River. Almost all of these surface areas are below approximately RM 217. Most of this surface is probably related to relatively flat-lying Columbia River Basalts that rim essentially the entire Oregon drainage divide of Hells Canyon, but only begin to rim the Idaho side at about RM 220 (Table 5.7).

E.1.2. Slopes Between 10 and 30 Degrees In the 10- to 30-degree slope class, Divide Creek has the most surface area. Approximately 42 percent of the total surface area of Divide Creek is in this slope class and more than 50 percent of the surface area of Divide Creek has a slope of less than 30 degrees. This would normally be correlated with age/degree of weathering, but in this case it is also probably a result of the large expanse of relatively flat-lying Columbia River Basalts in this drainage basin. Almost 90 percent of the alluvium surface area is less than 30 degrees (Table 5.7), but 53 percent is in the 10 to 30 percent slope class which is probably related to a relatively large alluvium surface area related to landslides and relatively small alluvial footprint of the Snake River. The surfaces of Bonneville terrace gravels also have a higher percentage in this slope class (46 percent) than in the less than 10 percent slope class (27 percent), which may be related to their steep side slope adjacent to the Snake River.

E.1.3. Slopes Between 30 and 40 Degrees Downriver of RM 217, surface areas with this slopes between 30 and 40 degrees cluster somewhat near the river. In general, surface areas within this slope class are particularly widely dispersed in the lower half of the drainage basin below about RM 217, but this slope class is also common in the tributaries of the upper half of the basin. The widespread distribution of this slope class indicates that a considerable amount of sediment is stored within the tributaries awaiting transport into the Snake River. The basin does not appear to be supply limited. Field observations from walking up several of the tributaries confirmed a general prevalence of sediment stored within the tributaries apparently awaiting sufficient surface water flow to transport the sediment into the Snake River. Above about RM 217, this slope class clusters into a medial to headwater drainage basin position. Since this slope class is typically associated with sediments achieving an angle of repose, this distribution would indicate that a considerable surface area of stored sediment is relatively evenly distributed in the lower part of the drainage basin on scree slopes and talus piles within individual tributary drainages and adjacent to the Snake River. However, in the upper part of the drainage, sediment is stored in the medial and upper part of the individual drainage basins, which are relatively more isolated from the Snake River. With the exception of alluvium and glacial deposits, this slope class does not distinguish between lithologic units. This slope class accounts for 10 to 20 percent of the surface area of all lithologic units with terrace gravels and landslides tending toward 30 degrees and Columbia River Basalt tending toward 40 degrees. This slope class accounts for less than 10 percent of the alluvium and glacial deposit surface area.

E.1.4. Slopes Between 40 and 60 Degrees By far the most dominant slope class in the drainage basin, this slope class indicates bedrock outcrops dominate the Hells Canyon drainage ranging from 30 percent (Wolf and Divide creeks; Table 5.6) to 65 percent (Big Canyon Creek). The widespread extent of this slope class is not unexpected, given the physiographic descriptions of Hells Canyon. However, this slope class also appears to represent a significant classification break point at RM 220 (Figure 5.15). Upstream from about RM 220, slopes greater than 60 degrees account for most of the remaining surface coverage. Downstream from RM 220, slopes less than 40 degrees account for most of the remaining surface area for this slope class. This implies that above about RM 220 there appears to be more recent, significantly higher geological uplift than below about RM 220. This slope class includes 50 to 60 percent of the surface area for Columbia River Basalt (49 percent), metamorphic rocks (52 percent) and intrusive rocks (59 percent). Most of the landslide surface area is in this slope class (42 percent). Finally, almost all (81 percent) of the glacial deposits occupy this slope class.

E.1.5. Slopes Greater Than 60 Degrees Most of the surface area of Hells Canyon with slopes greater than 60 degrees are located adjacent to the Snake River in the lower parts of individual drainage basins above about RM 217. This slope class is associated with the scenic high canyon walls on both sides of the canyon above about RM 235 and mostly on the eastern side of the canyon between RM 235 and RM 217. This slope class also extends into the mainstem from the downstream areas of Deep, Granite, and Sheep creek channels, rather than at the headwaters of these individual drainage basins. These relationships indicate that there has been, and probably continues to be, more rapid downcutting in the upper part of the Hells Canyon than in the lower part of the canyon. Rapid downcutting (high slope classes) is probably responsible for the linear character of the Snake River and most tributary drainages because the surface water follows and cuts along linear weaknesses in the bedrock such as faults, joints, and fractures. These relationships indicate that the Snake River, particularly in the upper end of Hells Canyon, tends to have a lesser supply of readily available sediments compared to the lower part of the canyon.

Metamorphic rocks and landslide deposits (most of which involve metamorphic bedrock) have the largest percentage of their total surface area of this slope class with 22 and 21 percent, respectively, compared to other lithologic units (Table 5.7). Almost three quarters of the total surface area of metamorphic rocks have slopes greater than 40 degrees. Glacial deposits have 11 percent of their total surface area in this slope class and 92 percent of glacial deposit surface area has slopes greater than 40 degrees. Intrusives have 9 percent of their surface area in this slope class, and Columbia River Basalt only 3 percent.

E.1.6. Slopes Classification Summary The slope classes and field observations document that a considerable amount of sediment is currently accumulating and has been accumulated during recent geologic history within the tributary drainages. The amount of sediment accumulating in the tributary drainages increases below about RM 220 but is still common in the tributary drainages above RM 220. This indicates that sediment transport out of the tributary drainages into the Snake River is not adequate to keep up with sediment supply. The prevalence of higher slope classes above about RM 220 suggests that this part of the Snake drainage basin is and has been experiencing higher structural uplift than the that below about RM 220. The Snake River, responding to this structural uplift, is rapidly downcutting into the dominantly metamorphic and intrusive bedrock, creating the steep walls of scenic Hells Canyon. Although this uplift likely produces relatively higher sediment volumes as the system tries to reach equilibrium, the high energy and transport capacity associated with the steep slopes appears to be larger than the sediment production.

E.2. Rock Varnish Analysis E.2.1. Indicator of Sediment Transport Color aerial photographs at a scale of 1:8400 taken August 9, 1997, were used to interpret physical structures and rock varnish coloration to investigate locations in the Snake River Canyon below Hells Canyon Dam where significant amounts of sediments are currently or were recently (geological time scale) contributed to the river (Photo 11). Pertinent examples of this is shown in Photos 11, 12, and 13. The highest contributors of sediments have probably been landslides. Older landslides commonly have a planar surface coated with a varnish tone about equal to that of surrounding bedrock and have been incised to variable degrees by erosion. In contrast, younger landslides have a well- defined planar surface that is usually at a steeper angle than older landslides. These younger landslides either have no coating or are coated with considerably less dark varnish tone than surrounding adjacent bedrock. In addition, younger landslides essentially have no varnish where they are actively shedding sediments to the river.

Rock varnish tonal colors indicate a relative age of sediment disturbance or movement. The color of surfaces with currently mobile sediments depends on the lithology, ranging from very light tan to white for most meta-sedimentary rock types and diorite, but dark gray to black for basalt and mafic metamorphic rocks. Bedrock, scree, talus, and soils that are and have been in stable physicochemical conditions (no movement by either natural or anthropogenic processes) have a consistent rock varnish coating that ranges from yellow-brown to orange in color similar to surrounding bedrock.

Rock varnish is a orange, yellow-brown, brown, and black coating that ubiquitously occurs on rocks, sediments and soils exposed to the dust, aerosols and other airborne material in the atmosphere (Kinsley 1998). Although rock varnish is found in all climatic conditions and all lithologies, it is most evident in arid to semi-arid environments. Under moist conditions such as the black rock varnish on rocks adjacent to the Snake River, it can form in a few decades (Liu and Broecker 2000). On the semi-arid to arid slopes of the canyon yellow-brown to orange rock varnish probably requires about “3,000 to 5,000 years to form a visually discernible patchy varnish and 10,000 years for a heavily coated varnish.”

The oldest varnish reported in the literature is on an alluvial fan surface in Death Valley with an age of 250,000 years (Liu and Broecker 2000) but very little varnish survives more than 100,000 years because of physical erosion (Kinsley 1998). Although scientific understanding of how varnish forms has wavered between abiotic to biotic for well over 100 years, recent technology suggests that varnish is formed by a combination of atmospheric material and microbes. The coating is made up of alternating laminae of dominantly manganese and iron oxides and oxyhydroxides (averaging a total of 30 percent) and clays (illite, smectite and chlorite) containing aluminum, silica, magnesium, potassium, and calcium as major ions. Fleisher et al. (1999) investigated rock varnish on samples from the Boise-Twin Falls area (in addition to other locations) and reported on the uranium, thorium, lead-210, and cesium-137 found as part of the trace elements in the coating. They report varnish growth rates of 1 to 15 micro-meters per 1,000 years. Varnish grows most rapidly in micro-depressions in the rock and slowest on a smooth surface between microdressions resulting in a patchy character to younger coatings. Thickness ranges from atomic layers to over 400 microns (0.4 millimeter) but most coatings are less than 300 microns thick.

To be conservative, a surface without varnish tonal color was considered to have been disturbed by either natural or anthropogenic processes in the last 1,000 years. In reality, some of the disturbances could have occurred from slides and slumping sediment movement just before the aerial photograph was taken. Most of the anthropogenic disturbances have probably occurred in the last 100 years based on the literature estimates of the minimum rate of varnish formation (Fleisher et al. 1999, and Liu and Broecker 2000).

Shiny black rock varnish on the variably moist rocks at and just above the common Snake River water level is a relatively thick manganese-oxide-rich varnish that can commonly form in about 10 to 25 years. This varnish occurs on steep- to very steeply dipping bedrock surfaces immediately adjacent to the river. The varnish at the base of the coating (within the fluctuating Snake River water level) is commonly dark brown, which is the color of a light coating of manganese oxide. This shiny black rock varnish on the canyon walls should not be confused with the abundant wet shiny black cobbles and pebbles in river bars, which result from river transport and polishing by finer-grained particles rather than varnish. Dorn and Oberland (1981) reports that the manganese- oxide-rich shiny black rock varnish forms in a moist, near neutral pH, aerobic condition, on a surface low in organic carbon; that is, a habitat suitable for mixotrophic manganese- oxidizing bacteria. The bacteria adsorb clays and these clays provide both inorganic and organic nutrients to the bacteria, while also mediating physicochemical environmental conditions that protect the bacteria from complete desiccation and high temperature.

Surfaces on the bedrock, sediment and soil above the shiny black varnish on bedrock adjacent to the river are variably colored yellow-brown to red by rock varnish. This rock varnish may extend down to the river level bars where black varnish does not occur. This varnish type that coats all these slopes has a higher iron and much lower manganese content than the black varnish and forms much more slowly. Yellow-brown is the dominant color of iron oxyhydroxides (goethite) and red is the dominant color of iron oxides (hematite) with relatively low manganese content. In general, the yellow-brown coating becomes the orange coating with time as the hydroxide is converted to oxide. It forms under less moist and more alkaline conditions than the black varnish but has essentially the same chemistry and mineralogy with proportionally less manganese.

E.2.2. Color Aerial Photograph Interpretation Table E-1 lists the river mileage of features identified on the aerial photographs that were considered relevant to sediment mobility in Hells Canyon between Hells Canyon Dam (RM 247.4) and RM 178.6, just below the Scenic River Boundary. The river miles for each notation have been scaled from the aerial photographs which were marked with individual river miles. Other sources for river mile data and feature descriptions included the 1999 Recreation Use Study (IPC 1999), Vallier (1998), and White and Vallier (1993). In general, the color of the varnish in Hells Canyon is generally darker on the easterly and northerly facing slopes and lighter on the westerly and southerly facing slopes.

The term “scree” is used in this appendix to mean a sheet or chute of loose, fragmental rock debris lying on or mantling a slope or hillside that commonly exceeds 40 degrees (Gray et al. 1974) formed chiefly by gravitational falling, rolling or sliding rock debris. The term “talus” refers to an accumulation, “heap or pile of rock fragments derived from and lying at the base of a cliff or very steep rocky slope” (commonly below scree) “considered as a unit and formed chiefly by gravitational falling, rolling or sliding” (Gray et al. 1974) rock debris. As described earlier, talus surfaces typically have a slope near 35 degrees because that is the average angle of repose.

Photos 4 and 12 show how the hillslopes in Hells Canyon are actively shedding sediments over the long-term. A total of 43 locations of mobilized sediment are described in this appendix with approximately half of the locations on the eastern (Idaho) and half on the western (Oregon) side of Hells Canyon. However, the upper Snake River reach (RM 247 to about RM 235) with the highest river gradient has 12 locations with eight (67 percent) on the eastern (Idaho) side and four (33 percent) on the western (Oregon) side of the river. The higher number of landslides on the eastern side of the canyon is probably related to the metamorphic bedrock along this side of the canyon with slopes exceeding 60 degrees. A relatively large sediment source is located between RM 242.7 to RM 243, where a composite landslide continues to provide sediment to the river in many geomorphic forms. Even though the western locations are fewer in number, they include the large Waterspout Landslide (RM 234 to 234.9), a major current and geologic- historical source of sediment to the Snake River (Photo 13). The confluence of both Stud Creek and Battle Creek with the Snake River suggest that at one time these creeks may have had a southerly flow rather than the current northerly flow.

There is a major change in lithologies at RM 235 from layered meta-sedimentary bedrock to dioritic intrusive. Intermediate gradient Snake River, beginning at about RM 235 to about RM 217, has 11 locations shedding sediments; six (55 percent) on the eastern side and five (45 percent) on the western side of the river. Rush Creek landslide between RM 231.4 and RM 231.9, is a major contributor of sediment in this river segment. However, this river segment also includes the High Bar and Alum landslides between RM 226.7 and RM 227.4 along with well-developed Bonneville terraces (particularly well preserved between RM 223 to RM 225). Bonneville terrace gravels are being actively eroded into the Snake River in three segments: between RM 233.1 to RM 233.2; RM 230.5 to RM 231; and more slowly between RM 223.5 and RM 223.9. This latter, more slowly eroding segment was probably initiated by and continues to be perpetrated by an fluvial debris fan created by Temperance Creek. Most, if not essentially all, of the sediment in the Temperance Creek Debris fan is from a relatively large and old northeasterly facing landslide within Temperance Creek. Most of the remaining sediment transport to the Snake River is from numerous scree slopes from both sides of the canyon.

An active bowl-shaped landslide occurs at about RM 214.9 across from Pittsburgh Landing and is included in the numbers of the above intermediate gradient suite because, with few exceptions, the landslides below this point appear to be older than those above. This landslide is a composite landslide made up of several blocks that move somewhat independently. The lower and particularly the southern-most block is moving more rapidly than the upper part of the landslide. The trail to the Pittsburgh Administration Site (across the river from Pittsburgh Landing) cuts across this southern-most mobile block. White and Vallier (1993) include this landslide and another much larger one on the intermediate slopes to the north of the river in a generalized geologic map of the Pittsburgh Landing area. Both landslides occur at and beneath a thrust fault thrusting Pennsylvanian-Permian Cougar Creek Complex rocks over the Jurassic marine sandstone and mudstone unit. Both landslides occur in the Jurassic unit, which in turn is being thrust over an older Jurassic conglomerate and sandstone unit. The landslide therefore occurs in a zone of isoclinal folding between two thrust faults. There is little doubt that these structural characteristics make a significant contribution to the formation of both landslides.

The Snake River makes a significant change from northeasterly flow to northwesterly flow at about RM 217 and below this point the river has a lower gradient. This lower gradient segment of the river has 20 locations actively shedding sediments; nine on the northern (Idaho) side (45 percent) and 11 on the southern (Oregon) side (55 percent) of the river. The color tone of the varnish and level of erosion on the landslide surfaces along this segment of the river suggest that these landslides are older than those occurring in the above this segment. Lithology may be contributing in part to this differentiation because this segment of the river, particularly within the uppermost half of this photo-interpreted river segment, is comprised mainly of Tertiary basalts. This lithology may be responding more rapidly to weathering than the largely metamorphosed rocks dominating the upper two river segments with higher river gradients. However, only two of the 20 locations occur within the basaltic rocks and the remaining 18 occur in other rock types dominated by older metamorphic suites. Therefore, lithological differences are probably not as important as structural differences in the landslide processes. This is further enforced by the significant landslides between RM 207 and RM 208 within meta-sedimentary units on the western side of the river. The slides form triangular-faceted surfaces on several slopes suggesting that dip-slope failure caused the landslides. The river erodes and undercuts the base of rocks dipping toward the river which then results in slope failure into the river as a sudden event. The sudden drop further fractures and breaks the falling bedrock into large blocks and fragments, commonly forming a rock fragment breccia surface on the eroding face, all of which the river can then more easily erode and transport, eventually forming a triangularly faceted surface that continuously sheds sediments to the river.

Multiple landslides occur between RM 208 and RM 178.6, most of which have a color tone to the varnish that matches the color tone of the surrounding rocks, suggesting stability of the landslide surfaces for a very long time. Additionally, many have undergone a significant amount of erosion (strongly incised drain lines cutting through the surface) to the point that many no longer have a clearly defined landslide surface. These characteristics suggest that these landslides are older than those in the above two river segments. However, one exception occurs between RM 189.2 and RM 189.7. Landslides in this segment are a series of dip-slope landslides beginning with the largest one at RM 189.7, decreasing in size to RM 189.2. All of these appear to be continually shedding sediments from their surfaces into the river. The landslide at RM 189.2 is a bowl-shaped landslide that appears to be actively moving as essentially a single block. In addition to landslides, relatively large areas of higher bedrock slopes shed sediments forming active scree slopes and talus piles.

Rock varnish signatures indicate variable sediment transport into the Snake River below Hells Canyon Dam. Episodic small to massive landslides have occurred along both sides of Hells Canyon and have obviously produced prodigious amounts of sediment into the Snake River over recent geologic history. In addition, the landslide surfaces continue to shed sediments into the river. Two landslides are currently active; one at about RM 214.9 and another at about RM 189.2. Relatively smaller landslides have occurred within tributary drainages. Varnish coatings indicate that landslides are younger above about RM 220 than those below.

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