The kame terraces of the Okanogan Valley are Alaskan in scale, but treeless and fully exposed. This extensive multi-level flight is continuously exposed between Brewster and Osoyoos, a distance of 100 km.
The terraces record voluminous ice-marginal sedimentation during the final downwasting of the Cordilleran Ice Sheet (Okanogan Lobe) from Eastern Washington. They are composed of a mix of outwash gravels, till, lacustrine sediments, and boulders. Tread surfaces are paired in places, unpaired in others. Surface connections between terrace systems in the Methow, Sanpoil, and Upper Columbia are certain to be made. Fields of kettles dot many surfaces as do large erratics (mostly basalt, granite). Local outburst fans and other curiosities exist.
Mapping the Okanogan terrace complex would involve covering a swath roughly 10 km wide x 100 km long (portions of 19 quadrangles). The valley widens locally to ~15 km at Omak, Brewster, Tonasket and the maps should as well, but the train of glacial sediments is largely confined by bedrock below 750m elevation (~2500'). Standard nomenclature for surficial units would cover what's here. Its a great project that I began in the early 2000s using standard topo sheets, but would love to tackle formally given that full lidar coverage is now available.
Several reasons for mapping the Okanogan Valley's glacial geology now:
- Okanogan kame terraces merge with, but are separate from The Great Terrace of the Columbia (Russell, 1898). At even the most rudimentary level, the framework of ice-marginal sedimentation - best expressed in the kame terrace complex - has yet to be worked out despite extensive mapping of glacial deposits done immediately to the east (Kovanen and Slaymaker, 2004; Lesemann and Brennand, 2009) and west (Barksdale, 1941, 1947, 1948, 1975; Waitt, 1972).
- Improved understanding of Okanogan Lobe vs. Puget Lobe activity during deglaciation (Hendy, 2009 in GSA Today), including the controversial but developing "subglacial underburst" story (Shaw et al., 1999; Lesemann and Brennand, 2009; Dawes, 2017).
- The terrace complex lies a the junction between Cordilleran Ice Sheet, mixed strata in proglacial Lake Columbia, and the subaerial Channeled Scabland to the south. Whatever the geologic analog to landscape ecology's "ecotone" is, this place is it.
- The Okanogan landscape remains a Pleistocene landscape. Its dry climate and sparse population has kept the area mostly undeveloped or under orchards. Landforms from the end of the Ice Age persist and are readily mappable. Fine-scale features such as Quaternary faults, local valley outburst fans (Johnson Creek Fan at Omak), and subtle swales literally jump out on lidar imagery.
- One of the larges and most detailed soil surveys ever produced was for the 1.4 million acre Colville Indian Reservation (Soil Survey Staff, 2002), which borders the Okanogan River for >50 km. Detailed geologic mapping would complement each other and likely spin off land management-related research ideas (wildfire, fisheries, forest productivity, ash-caped soils, etc.).
- The Colville Indian Reservation occupies a broad swath of the valley and has major interest in water resources of the Columbia-Okanogan River corridor. Partnership and expanded funding opportunities may present themselves.
- The terraces, in places, pose a landslide hazard to residences. While not particularly acute in the Okanogan Valley (compared to the Upper Columbia Valley in the vicinity of Kettle Falls-Seven Bays), historic and active slide areas should be formally delineated through detailed mapping.
- The Okanogan County Museum has housed an active and enthusiastic group of amateur geologists who have amassed an impressive archive of local geological knowledge. They are friendly and helpful without being annoying. They know this country well.
- The time is now to map the Okanogan Valley. Cheap land prices, year-round sunny weather, western vistas, and ever-increasing pressure from retirees from away will certainly change the place in coming decades. Geologic mapping, and especially surficial mapping, will benefit planners tasked with managing future growth.
The Okanogan kame terrace complex constitutes portions of 19 1:24,000-scale quadrangles in north-central Washington. The Valley is entirely within the boundaries of the Oroville and Omak 1:100,000 quads (#90-11, #90-12). However, the ribbon-like map area is not a huge area and is readily accessible by the roads network. Its an easy place to work.
Here's one of my draft field maps from 15 years ago. I found 5 to 6 terrace levels in the Okanogan Valley. My mapping covered the whole valley, but haven't searched my storage unit yet. The way I started mapping kame terraces was to identify the risers (escarpments) instead of the flats (treads). Risers are yellow in the map above. I approached it this way because a.) the terraces slope gently south so you couldn't just map surfaces on contours, b.) there are more than 5 different tread levels - sometimes more - and they don't always merge intuitively, c.) the back edges of terraces commonly merge with low-angle alluvial fans spilling from hillslopes behind, the line between is not apparent on aerial photos or in the contours (it was in the field), d.) I only had 1:24k scale topos and 30m DEMs to work with which were at a coarser resolution than the scale of important features, e.) mapping risers just worked a lot quicker and better than mapping the flats. At first, my goal was just to figure out the framework elements, which mapping risers allowed me to do efficiently.
Terraces appear to be paired across the Valley at the resolution of standard 1:24K quads. Does lidar show the same thing? A difference of a few meters of elevation is significant when you're talking about streambeds and stream gradients.
Are there several erosional bedrock surfaces (strath terraces) along the Valley in addition to the kame terraces (fill terraces)?
How lidar-mapped terraces compare to Kiver and Stradling's work on terrace elevations from quads (Kiver et al., 1989)?
Long profiles of ice-marginal bedrock channels would be cool.
Cross-valley elevation profile transects on a lidar DTM base drawn at an interval of rivermiles (or 1/2 rivermiles) along the Okanogan River would show the terrace system in detail.
Ash-influence soils of the Pacific Northwest by McDaniel and Hipple (2010). Ash-capped soils in the Okanogan Valley (black box is proposed mapping area) and adjacent Okanogan Highlands underlie some of the most productive forest lands in Washington State.
An excerpt from a large water rights/irrigation feasibility study I worked on with my former boss at the Colville Tribe. I made this and several others. like it. All depicted Tribal lands bordering the Columbia, Okanogan, and Sanpoil Rivers suitable for agricultural development based on soil properties, slope, pumping constraints, and other factors (green areas on map). I think they were used by the Tribe, hopefully for some productive purpose.
I created this soil catena figure to help non-soil experts working in the Okanogan Valley better understand where different soils occur in the landscape. The figure shows soil groups (Landtype Association Units) relate to landscape position and the relative abundance of each. Landtype Association Units are a combination of soils, topography, and geology developed using GIS data layers. LTA mapping was fashionable in the 1990s-early 2000s (i.e., ECOMAP terrestrial ecological unit inventories of Bailey and McNab) on USFS and NPS lands where detailed soils/surficial geology data was not available. LTA units represent the "durable elements" of the landscape at a scale relevant to land management work. I found this kind of "big picture" diagram proved helpful as a supplement ito the more technical Soil Survey document and in securing post-wildland fire rehab funding and other watershed-related grants. The kame terrace system of Okanogan Valley lies mainly below about 2500' elevation and include Upland Outwash Basins, Low Elevation Tills, Terrace Complex, Pleistocene and Holocene Alluvial Fans, and Alluvium of the Okanogan River and its tributaries.
I created this geology-soil index map for the Colville Indian Reservation. It shows major geologic units (defined by mining resources work), faults, and soil "type pedons" (locations where soil type sections were identified and described). Maps like this are not included in NRCS Soil Surveys, though they should be. The map helped staff get a better feel for where in the landscape certain soils were defined.
The Great Terrace of the Columbia was first identified by Russell (1898, 1900). Reconnaissance work in glacial deposits along former ice sheet margin was, often by horseback, documented in papers by W.C. Alden, H. Bretz, T.C. Chamberlain, W.L. Dawson, R.F. Flint, O.W. Freeman, G. Gibbs, W.H. Hobbs, F.O. Jones, C.R. Keyes, E.T. McKnight, O.E. Meinzer, G.E. Neff, J.W. "Karl" Oestereich, R.D. Salisbury, H.T.U. Smith, W.D. Thornbury, D.E. Trimble, and A.C. Waters. Many of the landscapes they sketched, photographed, and described remain accessible today.
Back when I taught for Boise State Geosciences (2010-2015), a student and I developed this looped technique for extracting terrace surfaces from DEMs using a combination of ArcGIS and program R. We successfully tested the methodology on Okanogan Valley kame terraces. We suspected that some of the surfaces were too subtle to see in the field or in aerial photos, but could be teased out of the DEM data if we could get the parameters right. This image shows our first results with hand-written notes. We improved the resolution on individual surfaces in subsequent model runs. Pixels shown in the chart slope 13% or less and are within ~1km of the river's thalweg. Colors indicated distance from river channel (warm=near, cool=far). Yellow and tan pixels are bedrock channels and channels incised in till located above most of the terraces.
Kiver and Stradling's work on terraces along the former glacial margin (Kiver et al., 1989). Okanogan Valley ("Omak Trench") is at left, but most of their work (several publications) occurred east of here.
And history happened in the Okanogan, so we've got that going for us, too.