The Imnaha Subbasin Summary has been developed as part of the rolling provincial review process developed by the Northwest Power Planning Council (NWPPC) in February 2000 in response to recommendations by the Independent Scientific Review Panel (ISRP) and the Columbia Basin Fish and Wildlife Authority (CBFWA).
This summary is an interim document that provides context for project proposals during the provincial reviews while a more extensive subbasin plan is developed. The Imnaha subbasin is one of 56 subbasins included within the Columbia basin (Figure 1). The process of developing a subbasin summary for the Imnaha subbasin was initiated as part of the provincial review process at a March 28, 2001 meeting in La Grande, Oregon. The short timeline for this project precluded the possibility of collecting new data. Instead this document summarizes existing documents, data not yet published in previous documents, and best professional opinion when reliable data was unavailable.
This document has been reviewed at several stages in its development by professionals familiar with the subbasin, and by state and federal agency personnel responsible for fish and wildlife in the region. A technical working group met several times in April and early May to review and contribute to the document. Without their help, this document could not have been written in such a short time.
This document forms a foundation for future assessment and planning efforts in the subbasin. It is our hope that this summary will enable those working to protect and restore fish and wildlife in the subbasin to move forward to fill data gaps and more effectively implement projects without needing to intensively research and integrate past data.
Figure 1. Location of Imnaha subbasin in the Blue Mountain Ecoprovince
The Imnaha River subbasin is located in the extreme northeast corner of Oregon and drains an area of 980 square miles (Figure 2). The mainstem is formed by the juncture of the North and South Forks at an elevation of 5,300 feet and flows in a northerly direction for approximately 63.5 miles to its confluence with the Snake River at river mile (RM) 191.7. Although several spring-fed tributaries occur in the subbasin, the Imnaha hydrology is primarily snow-melt dominated. The entire drainage is contained in EPA Reach 17060102.
The Imnaha River subbasin lies entirely within the Wallowa-Snake physiographic province and is characterized by majestic peaks, high tablelands, and deeply incised valleys. Elevations range from nearly 10,000 feet in the Wallowa Mountains to 975 feet at the river's mouth, while the plateaus, such as Lord Flat Plateau, rise to nearly 7,000 feet. Plant associations and climate vary with the topography and geology of the region.
The Wallowa granite is part of the Cretaceous/Jurassic (160-120 ma) Idaho batholith system (Vallier and Brooks 1987) (Figure 3). The weather-resistant granite now forms the high peaks of the Wallowa Mountains with nine peaks over 9,000 feet in elevation (Weis et al. 1976). Here the headwaters of many intermittent creeks begin in the U shaped valleys cut by Pleistocene glaciers (Wallowa-Whitman National Forest 1998). The tributaries merge at terminal moraines of crushed rock and fine sediment to form the Imnaha River and the Big and Little Sheep Creeks. The moraine sediment is a possible source of sediment during spring runoff.
The Imnaha River flows east out of the Wallowa Mountains towards Idaho, cutting through the fossiliferous Martin Bridge limestone and the Hurwall siltstone formations. Big and Little Sheep Creeks flow due north near the town of Joseph, Oregon through these formations. Cobbles of lime rock line the river and creek beds through this section of the subbasin (Wallowa-Whitman National Forest 1998). The Imnaha River begins to turn north as it cuts through the Clovercreek Greenstone near the Coverdale Campground. This bedrock consists of metamorphosed sedimentary and volcanic rock (Vallier 1973).
As the river turns north near the Coverdale campground, it begins cutting through many layers of lava from the Imnaha and Grande Ronde members of the Miocene (17.5-15.6 ma) Columbia River basalt group (Hooper and Swanson 1990). Deep V-shaped valleys form as the Imnaha River and its tributaries begin incising through the overlying and more durable Grande Ronde basalt that makes up the many cliff-faced columnar exposures on steep slopes. Dry, Crazyman, Summit, and Freezeout Creeks begin in this basalt to the east along Summit Ridge before entering the upper Imnaha River.
The underlying Imnaha basalt is more easily weathered and is easily recognized by the shallower slopes that often are mantled with deeper soils with fewer columnar basalt outcrops (Art Kreger, USFS Soils Scientist, personal communication February 8, 2001). Big Sheep and Little Sheep Creek valleys have a similar geology and morphology through the Columbia River basalt lava layers and join together in the Imnaha basalt in the central part of the subbasin. Trail and Camp Creeks start in Grande Ronde basalt on Zumwalt Prairie in the northwestern part of the subbasin. They empty into Sheep Creek and together these tributaries drain the western part of the subbasin into the main river channel at the town of Imnaha.
Quaternary alluvial deposits form narrow river terraces along the banks of the Imnaha River and it’s major tributaries. The alluvium contains river rock from upstream, colluvial basalt from the canyon side slopes, and Mazama ash and windblown silt mixed in with the soils that formed on the river terraces. These terraces are found in the central part of the Imnaha River and lower Big and Little Sheep creeks where the main channels have some ability to meander through the unconsolidated sediment. A recent study found that 84 percent of the riverbanks in the subbasin, including these terraces, are stable due mainly to vegetation and coarse sediment (Wallowa Whitman National Forest 1993).
North of Fence Creek down river from the town of Imnaha, the river begins to cut through older basement rock called the Wallowa Terrane (Vallier 1973). The Seven Devils Volcanic Group is a Permian-aged (290-240 ma) member of the Wallowa Terrane and is composed mostly of volcanic rock with sedimentary, and meta-sedimentary rock formations, and intrusive granites.
Horse, Cow, and Lightning creeks all start in the Grande Ronde basalt in the Lord Flat and Summit Ridge area along the eastern part of the subbasin. All three cut through the older Imnaha basalt and into the metamorphic rocks of the Wallowa terrene and experience similar river valley morphology before they enter into the lower Imnaha River’s canyon. The river and creeks are heavily channeled by the crystalline Seven Devils formation.
The late Pleistocene (15 ka) Bonneville flood came down through Hell’s Canyon past the mouth of the Imnaha River. This flood would have back-flooded the Imnaha River valley to some degree, but there is no sediment that can be accurately identified as having come from that one time flooding event (Mark Ferns, Geologist with DOGAMI, personal communication 2/8/01). As the Imnaha River enters into the Snake River through an alluvial fan of river-rock and sand, as well as tailings from early mining operations (Vallier 1998).