The Imnaha subbasin is made up of a broad range of elevation and topographic relief (975 – 10,000 feet and 0 to > 90% slopes) (Figure 4). The granite peaks of the Wallowa Mountains are barren rock slopes and cliffs ranging from 90% to vertical. The Martin Bridge and Hurwall formations have soils forming on 30 to 90% slopes in the higher elevations in the Imnaha drainage where the South, Middle, and North Forks converge in U shaped valleys on the eastern side of the Wallowa Mountains (Weis et al. 1976).
As the river turns north near Coverdale campground it begins cutting through the Grande Ronde basalt, forming a deep V shaped valley with the typical columnar basalt cliff faces on the steeper slopes (30 to 90%). This is often referred to as bench type topography (Tom Smith, NRCS Soils Scientist, personal communication February 8, 2001). The Imnaha river channel erodes through the Grande Ronde basalt and into the more erodible Imnaha basalt near the intersection of North Pine Road and the Imnaha River Road. The river valley begins to widen, forming the shallow valley slopes that typify the central part of the Imnaha River valley corridor. The shallow slopes range from 5 to 15% near the river and 15 to 30% near the canyon walls (Art Kreger, USFS Soils Scientist, personal communication February 8, 2001).
As the Imnaha River carved its way into the more durable metamorphic rocks of the Wallowa terrane, it became incised in near vertical canyon walls, with only enough room at the bottom for the riverbed itself and the well known Ni-Mi-Puu foot trail (Wallowa-Whitman National Forest 1998).
Figure 4. Topography and elevation in the Imnaha subbasin.
The climate may be described as temperate continental and dry, with the Cascade Mountains acting as a barrier to the coastal marine influence. Temperature and precipitation are greatly influenced by elevation. Mean summer temperatures below 3,000 feet are 80 degrees to 90 degrees Fahrenheit and mean winter temperatures are 30 F. Between 3,000 feet and 6,000 feet, the mean summer temperature is 61 F and the mean winter temperature is 20 F while above 6,000 feet the average temperature in July is 54 F and in January is 14 F. Precipitation below 2,000 feet averages less than 10 inches per year (mostly as rain) whereas above 8,000 feet the average is greater than 50 inches per year (mostly as snow). The variations in precipitation for the Imnaha subbasin are depicted in Figure 5. Because the data used in the figure is based upon neighboring climate station data and extrapolated to fit local topography and weather patterns, it may not be entirely accurate. Nevertheless, precipitation estimates range from nine inches per year at the confluence of the Imnaha with the Snake, to 75 inches annually at the headwaters (PRISM data).
A SNOTEL site is located in the neighboring Grande Ronde subbasin on Mt. Howard (elevation 7,910 feet). Data collected at the site includes average monthly precipitation and snow water equivalence, both of which have been summarized by the Natural Resources Conservation Service. As shown in Table 1 average monthly precipitation at the site is greatest during the months of November (5.7 inches) and March (5.7 inches), while average snow water equivalence is highest during mid-April (17.0 inches).
Table 1. Mt. Howard SNOTEL averages: monthly precipitation and snow water equivalence (SWE) (downloaded April 19, 2001. http://www.wcc.nrcs.usda.gov/wcc.html).
While the SNOTEL data provides a general idea of precipitation patterns in the subbasin, the climate regimes in the Imnaha are highly variable and are dominated by microclimates specific to aspect, location and region. North slopes tend to be wetter and cooler than south slopes. Stream bottoms provide a cooler damper climate than hillsides or ridge tops. Areas with good air drainage remain warmer in the winter than pockets with little or no air drainage.