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(E.1-2) (Appendix E) Geomorphology of the Hells Canyon Reach of the Snake River

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(E.1-2) (Appendix E) Geomorphology of the Hells Canyon Reach of the Snake River 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.
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