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Home , Cabinet Gorge Dam, Cabinet Mountains, Giant current ripples, Kootenay Lake, Lake Missoula, Latah Creek

Mapping the Deluge: Sandpoint to Cabinet Gorge ,

Roy M. Breckenridge Dean L. Garwood Jack Nisbet

Idaho Geological Survey University of Idaho Staff Report 14-1 875 Perimeter Drive MS 3014 August 2014 Moscow, Idaho 83844-3014 www.IdahoGeology.org Mapping the Deluge: Sandpoint to Cabinet Gorge Dam, Idaho

Roy M. Breckenridge Dean L. Garwood Jack Nisbet

Staff Reports present timely information for public distribution. This publication may not conform to the agency’s standards.

Idaho Geological Survey University of Idaho Staff Report 14-1 875 Perimeter Drive MS 3014 August 2014 Moscow, Idaho 83844-3014 www.IdahoGeology.org MAPPING THE DELUGE: SANDPOINT TO CABINET GORGE DAM, IDAHO

September 14, 2013

Roy M. Breckenridge*, Dean L. Garwood**, and Jack Nisbet***

*Professor Emeritus, University of Idaho **Senior Geologist, Idaho Geological Survey *** Spokane author

The information in this field guide is prepared for use by the Institute, Coeur du Deluge Chapter and should not be used for other purposes without written consent of the authors. This event is sponsored by the Ice Age Floods Institute, http://www.iceagefloodsinstitute.org/deluge.html

List of cover figures/photos x Part of David Thompson Map of North America, 1823. x Colorized version of figure by J.T. Pardee, 1910 x Colorized version of map by T. Large, 1921 x Colorized version of figure by J H. Bretz, 1923 x Photo by NASA John Phillips Pend Oreille ISS011E13607: on 26 September 2005 at 17:54:50 GMT from the International Space Station using a KODAK DSC760 camera with a 180 mm lens. x Part of Glacial Map of North Idaho, Roy M. Breckenridge and Dean L. Garwood, 2013

Chronology of exploration and mapping

1792 Robert Gray discovers mouth of

1793 Alexander Mackenzie travels across to Pacific Coast

1805 Lewis and Clark reach Pacific Ocean

1809 David Thompson builds trading houses on Pend Oreille and Flathead

1811 Thompson follows Columbia to Pacific and finishes survey of river’s entire length

1814 Thompson delivers first map of western North America

1815 William Smith completes Geologic Map of British Isles

1825-33 David Douglas on Columbia River and Plateau

1830 Charles Lyell publishes Principles of

1910 Joseph T. Pardee paper and map of Missoula

1921 Thomas Large Map

1923 J Harlan Bretz Map

1942 J.T. Pardee Unusual currents in glacial

1950 W.C.Alden Map (includes Pardee’s earlier map)

2005 NASA space station photography from Phillips

2013 Glacial Map of North Idaho by Breckenridge and Garwood INTRODUCTION This field excursion will explore the geology, physical geography, history, and culture of Northern Idaho. We will compare maps of the area both old and new at the Coeur du Deluge (heart of the floods).The route is from Sandpoint to Clark Fork and the Cabinet Gorge Dam to observe the glacial, , and lake features in the area of the ice dam. Geologists Roy Breckenridge and Dean Garwood with naturalist Jack Nisbet will guide us through a history of the geologic terrain and mapping of the landscape to present day.

Field stops will feature new geologic research on late glaciation of the Purcell Trench, character of the Clark Fork Ice Dam, and the geology of Lake Pend Oreille. Here we present a few selected examples of exploration and geologic maps with focus on the heart of the deluge in Idaho.

OVERVIEW OF GEOLOGIC HISTORY Bedrock Geology (Breckenridge and Lewis, 2005) Northern Idaho has a rich geologic history, the incomplete record of which extends from 2.6 billion years ago to the present (Figure 1). The oldest known rocks, exposed near Priest River, originated as sedimentary layers deposited in an ancient ocean. About 2.6 billion years ago, these deposits were intruded by granitic magmas and at some later point metamorphosed to form metasedimentary gneisses. The details of the subsequent billion years of geologic time are sketchy, but included granite magmatism west of present-day LaClede about 1.57 billion years ago. Our geologic record improves dramatically at 1.47 billion years when widespread deposition of the Belt Supergroup began, continued for at least 70 million years, and produced a tremendous thickness of that contains remarkably well- preserved mudcracks, ripple marks, and algal mats. The Belt rocks are widely exposed east of Sandpoint. The Belt was deposited in a rifted basin; the lower strata (Prichard Formation) were deposited in deep water and mafic sills intruded the Prichard shortly during and shortly after deposition. The upper strata (Ravalli Group and above) were deposited in a shallow sea, perhaps similar to the Caspian Sea. The geologic record is again limited until about 800-600 million years ago when continental separation occurred and coarse sediments and volcanics of the Windermere Supergroup were deposited. An ice age has been documented at this time as well. Shortly after this in the Cambrian, sediments containing trilobites were laid down. Remnants of the Cambrian strata are preserved in down-faulted blocks near the southern part of Lake Pend Oreille.

Mountain building began in the Cretaceous (about 140 million years ago) as the entire region underwent compression. Rocks in the west were thrust up and over rocks to the east and the crust was thickened. Strata low in the geologic column were buried to great depths and metamorphosed. was extensive, and sediments were shed eastward into central off the uplifted land mass. Granitic magmas intruded into the middle and upper parts of the earth’s crust and cooled slowly, forming batholiths. Most of the central and northern parts of the are underlain by this Cretaceous granite, and isolated granitic bodies intruded east and south of Sandpoint. In the Eocene about 50 million years ago, compression gave way to ENE-WSW extension, which allowed localized areas of the crust such as the Selkirk Mountains to rise, and the overlying rock was stripped away by erosion. This extension brought high-grade metamorphic rocks such as the Hauser Lake gneiss to the surface in the footwall of large low-angle normal faults (detachment faults). The Eastern Newport fault is one such fault, characterized by relatively low-grade Prichard Formation in the upper plate (hanging wall) and high-grade Hauser Lake gneiss and granitic rocks in the lower plate (footwall). The high-grade gneiss is thought to be roughly the same age as the Prichard Formation, just at a higher metamorphic grade. During the same time, more magma intruded, which formed dikes in the area northwest of Hope and a granite pluton at Wrencoe. Basins developed, and conglomerate was deposited in the area north of Sandpoint. North-south trending extensional faulting in the Purcell Trench also occurred at this time. The youngest

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Figure 1. Part of Glacial Geologic Map of North Idaho (Breckenridge, R.M. and D.L. Garwood, 2012).

2 bedrock in the area is the Miocene Columbia River Basalt Group, which flowed north toward Sandpoint about 16 million years ago. Exposures end just north of Athol. More extensive valley filling lava flows have presumably been stripped away by the .

Ice Age Floods

During the last ice age () a lobe of the advanced from Canada into the , blocked the mouth of the , and created Lake Missoula. At maximum extent this glacial lake was up to 2,000 feet deep and covered 3,000 square miles of . Catastrophic failure of the Clark Fork ice dam released more than 500 cubic miles of water at a rate estimated at 10 times the combined flow of all the present-day rivers on earth. The torrent of water, sediment and ice thundered across the states of Montana, Idaho, , and to the Pacific Ocean. The enormous energy of the floodwater carved the land into and cataracts, excavated more than 50 cubic miles of soil and rock, piled boulders in huge gravel bars, and drowned entire off stream valleys in beds of mud. The continuous southward ice flow at the terminus repeatedly blocked the Clark Fork River and refilled Lake Missoula. The latest episode of flood outbursts occurred from about 17,000 to 12,000 years ago.

In 1923, Professor of the University of Chicago began publishing a series of papers explaining the origin of the Channeled Scabland in . He attributed this system of dry channels, , and cataracts to an episode of flooding on a scale larger than geologists had ever recognized. Prominent geologists disputed his hypothesis, and the resulting controversy is one of the most famous in geologic literature. Bretz’s ideas for such large-scale flooding were viewed as a challenge to the uniformitarian principles then ruling the science of geology. J.T. Pardee of the U.S. Geological Survey first studied glacial Lake Missoula in 1910 but it was until the early 1940s before he presented evidence for rapid drainage of the large ice-dammed lake. He estimated the lake contained 500 cubic miles of water and drained at a rate of 9.46 cubic miles an hour. Pardee’s explanation of unusual currents and flood features in the lake basin provided Bretz with the long-awaited source for flooding in the Channeled Scabland. Recently geologists’ attention has mostly focused on the recognition of evidence for multiple floods and timing of the ice age events. Today the effects of the ice age floods can be observed in an area covering 16,000 square miles in Montana, Idaho, Washington and Oregon.

EXPLORATION AND MAPPING

Between 1808 and 1812, North West Company fur agent and surveyor David Thompson traveled through these ice and flood features as he established the first circle of trade houses in what his company called “The .” Thompson’s distinctive maps (Figure 2) clearly show that the trails Kootenai and Salish people showed him follow geologic features across the landscape. In spring 1808, Thompson and his crew canoed down the Kootenai River to an encampment near modern Bonners Ferry. From there Kootenai people led him north to Kootenay Lake, following the path of the Purcell Lobe as it retreated in the last ice age. Although the trader wanted to push south to Lake Pend Oreille, his guides told him that spring flooding had rendered the trail impassable. It was not until fall 1809 that the Kootenai led him south into the Pend Oreille Country. He established his Kullyspel House at the Clark Fork Delta, which during the would have been beneath the ice dam that backed up Lake Missoula. He wintered in Saleesh House, about 70 miles upstream on the Clark Fork at the modern town of Thompson Falls, Montana. In spring 1810, he dispatched clerk Jaco Finlay to build , a full hundred miles across a divide and downstream on the . Over the winter of 1811-12, Thompson visited and the area around Missoula Montana. On his maps, mountains rendered with the “caterpillar” method of the time define the outline of glacial Lake Missoula above the Clark Fork, Flathead, and Bitterroot Rivers. The tribal trail he called the “Skeetshoo Road” leading from Kullyspel to Spokane House follows ice age flood channels across Rathdrum Prairie between the Pend Oreille and

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Spokane drainages. At least two other major tribal trails on his maps, “the Kullyspel Road” and the “Shawpatin-Pilloossees Road” trace flood channels from the Pend Oreille all the way to the .

Figure 2. A part of David Thompson’s 1823 map of North America from 84 (deg) West (Sheet 7) includes the area covered by Lake Missoula, the Clark Fork Ice dam area, and dotted tribal trails. Public Records Office, Kew, England. Flathead Lake (“Saleesh Lake”) middle right; Pend Oreille Lake (“Kullyspel Lake”).

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FIELD TRIP GUIDE The field trip begins and ends in Sandpoint, Idaho. From Sandpoint the route follows Highway 200 to Cabinet Gorge Dam and returns to Sandpoint. A map of the trip route and stops is included on the back cover of this field guide. Some of the sites and features discussed are on private property and permission to trespass is not implied by the authors or sponsors. SANDPOINT CITY BEACH In September 1809, David Thompson and a voyageur named Joseph Beaulieu, guided by “a Kullyspel lad,” rode horses around the north end of Lake Pend Oreille to the river. Thompson described a “sandy point” near City Beach at modern Sandpoint, then continued to the outlet of the river near Dover. When he canoed across the lake in a Kalispel canoe in April 1810, Thompson put up at the “point of Sand.” When he canoed the route again in June 1811, he set a course by “the Rock below the Sandy Point.” The rock feature is now known as Tank Hill and is composed of granitic bedrock scoured by ice age glaciation and floods. A handful of artists followed in Thompson’s tracks and sketched the landscape (Figures 3 & 4).

Figure 3. On August 12, 1845, British Army officer Henry James Warre made a pen and ink sketch of the distinctive flood-carved feature (Grief Mountain) directly across from City Beach and titled it “Kuttispelm Lake.”

The Clark Fork ice dam and the Lake Missoula outburst area in northern Idaho remained essentially unexamined and largely unmapped by contemporary geologists until the Idaho Geological Survey began studying the surficial geology through USGS STATEMAP projects. This mapping in Idaho serves as a resource inventory for viewpoints and features of the Ice Age Floods National Geologic Trail. Geologic mapping includes areas of surficial deposits and landforms created by the Cordilleran ice sheet, alpine , and multiple catastrophic outburst floods. Field evidence, including paleomagnetic studies of lake and flood rhythmites, substantiates repeated failures of the ice dam and multiple outburst floods from LakeMissoula. This new mapping defines the extent of ice, glacial lakes, and routes of outburst floods.

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Figure 4. Topographic map illustrating Warre’s view and the field trip route toward Hope.

Figure 5. Close up of the geology of the Sandpoint area showing location of cosmogenic samples (Red star symbol).

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These data are critical for testing new models of glacial outburst floods and water movement in glaciers. The Quaternary deposits previously undivided on earlier maps are now subdivided into an established stratigraphic framework (Figure 5). Boulders deposited near the of the Purcell Trench ice lobe and post-flood recessional were sampled for cosmogenic surface exposure dating. Cosmogenic 10Be surface exposure ages (mean weighted) constrain the glacial maximum ice limit near the Clark Fork ice dam at 14.1 ± 0.6 ka (Breckenridge and Phillips, 2010; Figure 6). deposits near the -Canada border constrain the ice recession (10Be range) from 13.3 to 7.7 ka (W. Phillips, written communication, 2011). Locally, tributary valley glaciers of the Cabinet Range contributed to the ice stream. Glacial till, outwash, and lacustrine deposits accumulated in the valleys. After retreat of the continental ice, mountain valley glaciers persisted in the higher of the and the Cabinet and Selkirk ranges. Glacial outwash and flood deposits are the main aquifers in the Sandpoint, Coeur’dAlene, and Spokane region as well as the prime source of sand and gravel.

Figure 6 (to the right). 10Be exposure ages for the Purcell Lobe plotted with existing chronologies. Except where noted, ages are radiocarbon dates calibrated with CALIB rev 6.0.1 (Stuiver and Reimer, 1993). Ice Dam Duration shows duration of blockage of the Clark Fork by the Purcell Lobe as inferred by Atwater (1986) and Waitt et al. (2009). Glacial Deposits shows duration of ice sheet glaciation in southern based upon radiocarbon ages of advance and recessional deposits (Clague, 1980); and the duration of ice sheet glaciation in Yellowstone based upon 10Be ages (Licciardi and Pierce, 2008). Tephras from Cascade volcanoes provide age control for numerous flood and glacial deposits in the . Shown are recent age estimates for Mount St Helens S set and Peak G and B sets. Flood Deposits shows ages Missoula floods deposits from the Columbia Basin of Washington State.

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HIGHWAY GEOLOGIC OVERLOOK STOP

James Madison Alden was a 25-year old Bostonian hired as an illustrator by the American teams of International Boundary Commission. In 1860 he painted several watercolors sketches of Lake Pend Oreille that now reside at the National Archives (Figures 7 & 8).

Lake Pend Oreille in northern Idaho is a fascinating geomorphic feature. This lake, the largest in Idaho and deepest by far in the region, lies in a basin formed by Cordilleran glaciations immediately below the site of the ice that repeatedly formed Pleistocene Lake Missoula. The Cordilleran ice sheet extended farthest along major south-trending valleys and lowlands, forming several composite lobes Figure 7. Alden’s painting of view from Ponderay point to Antelope segregated by highlands and Mountain. mountain, leaving behind distinctive landforms, weathering, and soils in these valleys. The outflows of Lake Missoula created some of the greatest floods known to have occurred on earth. The Missoula floods gouged thousands of square kilometers of anatomizing river channels more than 12,000 years ago and created the dominant features of the present landscape on the . Events such as Cordilleran ice lobe Figure 8. Google Earth view from Ponderay point to Antelope Mountain. advances and Lake Missoula flood catastrophes are important to because they form the basis for speculation and contemplation on the role of infrequent but large scale and spectacular events on the shaping of planetary landscapes. Although the glacial deposits along the southern extent of the Cordilleran ice sheet have been well explored and the effects of the Lake Missoula floods have been studied in detail across most of the Columbia Plateau and down the Columbia River to the Pacific Ocean, little is known about the Lake Pend Oreille basin. This

8 bedrock basin owes its origin to glacial processes at the southern edge of an ice lobe and was the first pre- existing geomorphic feature that the catastrophic Lake Missoula floodwaters encountered.

The lake level is maintained ~2062 feet above sea level, with the surrounding terrain as high as 6000 feet. The maximum depth of the lake is an impressive 1150 feet, the deepest lake, by far, in the region and listed as fifth deepest in the continent. The location of the lake is probably related to an ancestral river valley controlled by faults. Lake Pend Oreille was carved repeatedly by a lobe of Pleistocene ice, scoured by ice age floods and filled with glacial till, outwash, catastrophic flood deposits, and post-glacial sediments (Figures 9 & 10).

Figure 9. The 1000 Hz seismic data across Lake Pend Oreille basin. The location is shown as X-X' on Figure 1. Roman numerals refer to seismic stratigraphic units discussed in the text. A water velocity of 1425 m/s was used to calculate water depths.

Figure 10. A 500 Hz seismic record section across Lake Pend Oreille basin in the vicinity of cross- section B-B'. Roman numerals refer to seismic stratigraphic units discussed in the text. On this section, a water velocity of 1460 m/s was used to calculate water depths.

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Data from United States Navy (geophysical) seismic reflection surveys provide more information on the origin and character of the lake basin. Although these surveys were conducted primarily for geotechnical studies of the lake-bottom surface, the data also reveal distinct sub-bottom reflections. The seismic reflection surveys show that the bedrock lake basin has been glacially overdeepened to a depth more than 500 ft (152 m) below present-day sea level. The great depth of the lake and the thickness of the lake bottom sediment make it difficult to core but available cores allows interpretation of the geophysical packages. The seismic sections are interpreted to show a record of subglacial erosion, Missoula Flood deposition, and a post-flood glacial readvance.

The seismic data of Lake Pend Oreille, obtained using towed acoustic sources and hydrophone receivers, were recorded graphically at different times using high frequency (about 1000 Hz) and low frequency (500 Hz) seismic reflection systems. The higher frequency data can resolve units as thin as 0.5 m, whereas the lower frequency records are capable of resolving reflections from bedrock as deep as 1 km below the lake surface.

Four seismic stratigraphic units are present. The lake sub-bottom includes a section of layered sediments consisting of an upper unit (I) of thin-bedded sediments and a lower unit (II) containing several packages of thicker stratified sediment. The net maximum thickness of stratified units I and II is 50 m, each unit being at most 25 m thick in the center of the lake. The boundary between units I and II is highly reflective, indicating a change in acoustic impedance and suggesting a change in sediment type or geotechnical attributes at that interface. A U-shaped boundary separates units II and III. This boundary appears to be somewhat irregular with concave downward diffraction patterns suggestive of rough topology and possibly represents a late glacial advance of ice over older sediment. Unit III, which includes over 99% of the basin sediment, has a seismic signature of massive to weakly stratified seismically transparent material. Based on the lake proximity to the Lake Missoula ice dams and on the surficial geology south of the lake, unit III is interpreted to represent Missoula Flood deposits. The contact of unit III with the bedrock unit (IV) is the margin of the glacially-cut valley. The net thickness of unconsolidated sediment at the deepest part of Lake Pend Oreille is about 490 m and the bedrock basin beneath the lake extends to 214 m below sea level (Breckenridge and Sprenke, 1997).

Seismic studies of the bedrock morphology and sedimentary facies of inland linear valleys, such as the Okanogan and Kalamalka valleys in British Columbia, have revealed bedrock erosion to depths well below sea level and subsequent rapid infilling by sediment (Gilbert 1975, Fulton and Smith 1978, Mullins et al. 1990, Eyles et al. 1990, Desloges and Gilbert 1991, Gilbert and Desloges 1992). These valleys, which are similar to Lake Pend Oreille basin, contain substantial sediment thicknesses and owe their locations to structural lineaments and their morphology to large-scale glacial erosion beneath Cordilleran ice sheets. Shaw and others, 1999, proposed that flood releases beneath the British Columbia Ice Sheet were a contributing source of scabland flooding. These are all elongate, deep valleys, controlled by pre- existing fluvial channels, preferentially carved along pre-existing structural features at the southern margins of the Cordilleran ice sheet.

SAM OWEN STOP

In early September, 1809, David Thompson and his crew, which included clerk Finan McDonald and several voyageurs, camped on this peninsula with 54 Flat Heads (who he also called Pend Oreille, Kullyspel, or Salish), 23 Coeur d’Alenes, and 4 Kootenai people (Figure 11).

September 10 A very fine day. Early set off with 2 Flat Heads to look for a place to build a House, we at length found a place somewhat eligible but Labours under the

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want of good earth. I returned & we got all the Goods embarked by the Flat Heads & landed the whole by 3 p.m., when we set up our Lodge & Tents &C.

The men were “cutting, hauling & squaring wood for the upper floor of the Warehouse” of the Kullyspel House trading post when Thompson left to explore downstream on September 26.

Figure 11. Close up of David Thompson map showing the peninsula and his “NWCo” script signifying a North West Company post.

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Ice Dam Characteristics

One of the most intriguing questions about the catastrophic flooding is how the ice dam failed. Various mechanisms for glacial outburst floods have been proposed: Ice erosion by overflow water, subglacial failure by flotation, deformation of ice by water pressure, and erosion of subglacial tunnels by flowing water. One model suggests a self-dumping phenomenon. In this mechanism, floodwaters are released when the lake level reaches nine-tenths the height of the ice. At this depth the ice becomes buoyant, subglacial tunnels form and enlarge, and drainage occurs until hydrostatic pressure is decreased and the ice again seals the lake. The self-emptying model is used to explain the numerous cycles in the rhythmite deposits and to interpret each cycle as a separate flood. Even so, only the total collapse of the ice dam can explain the largest of the catastrophic floods. Sub-glacial tunneling and enlargement due to thermal erosion progressing to collapse have also been proposed as well as catastrophic failure due to water pressure. All are dependent on the configuration of the ice dam and structure of the ice. Five or more ice and flood spillways pass through the Green Monarch Ridge (Figures 12 & 13).

Figure 12. View east of Clark Fork drainage, note spillways in foreground and strandlines in distance. Photo credit: 1933 Washington Air Guard.

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Figure 13. View of Green Monarch Mountain and the location of the ice dam at the eastern margin of Lake Pend Oreille. Photo credit: 1933 Washington Air Guard.

Rhythmites

Near the ice dam a 34-meter section of glaciolacustrine sediments is preserved in Lightning Creek, a tributary of the Clark Fork (Figure 14). The section is composed of alternating silt and clay rhythmites (varves) deposited in Lake Missoula. In order to correlate and compare the chronology of this record with other sections, Breckenridge and Othberg (1998) analyzed 155 samples from 31 horizons in the rhythmite section for paleomagnetic directions. The sediments record stable paleomagnetism and the direction vectors within single sampled units have a low dispersion. Inclinations appear too shallow, probably due to compaction. The declination curve shows a pattern suggestive of secular variation of the geomagnetic field. Uniform rhythmites in the lower 12 meters of the section indicate stable deposition and no more than a 10° variation in magnetic declination in a short-lived rapidly filling glacial lake. The upper 22 meters of rhythmites reflect different sedimentation, and a marked change in magnetic declination 20° to the east with greater variation between sampled horizons. Rippled sand beds representing lake-emptying cycles interrupt the upper section. Erosion during these high energy periods probably removed record and the section includes several hiatuses. Thus the upper section represents a longer time period accounting for greater secular variation. The existence of several emptying cycles also is indicative of smaller late

13 glacial flooding episodes and the possibility of ice marginal drainages at lower lake levels rather than complete catastrophic failure. Other field evidence at the ice dam including the preservation of this section near the ice dam in an area subject to flood erosion implies they record the final history of the waning ice dam and lake cycles. The variation in declination appears consistent with annual varves.

Figure 14. Glaciolacustrine sediments and secular variation curve for Lightning Creek (Breckenridge and Othberg, 1998).

An exposure of Pleistocene sediments in the flood path on Peninsula Road near Priest River, Idaho has been used as evidence for multiple outburst floods discharged from glacial Lake Missoula. This section of rhythmites, first described by E. H. Walker in 1967 and later by Waitt (1984) is composed of alternating silt/clay units and coarser units of sand. The fine-grained glaciolacustrine sediments have been interpreted as stable lake-filling cycles whereas the sand beds have been interpreted as lake-invading flood deposits from the periodic outbursts of Lake Missoula. Breckenridge and Othberg (1994) measured the section for stratigraphic reference and correlation with other localities. The 1-2 m thick glaciolacustrine units contain 20-50 sets of rhythmites (couplets) that are also finely laminated. Pebble- to boulder-sized dropstones are present. Laboratory analysis of particle sizes using a Coulter Counter shows the couplets are composed of a light-colored fine to medium silt layer and a dark clay layer. The silt and clay units were sampled for laboratory paleomagnetic measurements. The results show that the 218 samples from 10 rhythmite units record stable paleomagnetic directions. Dispersion of the direction vectors within sampled horizons typically is low. Inclinations appear too shallow, perhaps due to compaction. The declination curve shows a pattern suggestive of secular variation of the geomagnetic field with amplitudes of 10°-30°. Similar declinations persist across the beds of sand, suggesting rapid deposition. These data help confirm that the sand units were deposited nearly instantaneously compared with the rhythmite units, a conclusion which

14 is consistent with a model of multiple outburst floods. Based on varve counts at this site and at Lightning Creek, the filling episodes for Glacial Lake Missoula lasted 20-50 years. The results at Peninsula Road suggest, however, that filling episodes were ten times longer.

CABINET GORGE DAM VIEWPOINT STOP

On October 11, 1809, David Thompson explored upstream from Kullyspel House on the Clark Fork River:

River about 60 yards wide with a deep rapid Current. We went along the Beach composed of ugly bad Stones, ‘till 9 ½ AM

At 9.40 AM stopped at a strong Rapid where we found Tents of Saleesh fishing Herrings with a small dipping net—of these fish they take great quantities, they gave us about 20 of them for which I paid them a foot of Tob[acco]

On April 21, 1810, Thompson was returning downstream from his Saleesh House at Thompson Falls to Kullyspel House on Lake Pend Oreille when he again passed through Heron Rapid and Cabinet Gorge:

Snow in the Night—cloudy blowy Morng – At 5 ½ AM Set off Co[ourse] as yesterday + ½ M[ile] N 65 W 1/3 M S 85W ½ S 10 W ¼ SW 1/6 S 70 W ½ West ½ M First part with the Line then carried 450 yds on the right Side at the Herring Rapid [Heron Rapid] bad large Stones & Snow, but not so bad by many degrees as yesterday’s Portage the Snow bore us up pretty well – began at 5.50 AM & set off at 8 ¾ AM hav[in]g breakfasted S 88W 1/3 N 25 W 1/3 Wt ¾ R. S 85 W 2 M S 70 W 1 ¼ M S 25 W 1/5 S 15 E 1/6 these last 2 Co[urses] among winding narrow perpend[icular] Rocks on each Side [Cabinet Gorge]

Geologic Setting

Cabinet Gorge dam on the Clark Fork River was completed in 1952 by Washington Water Power Company now AVISTA. The 600-foot-long and 200-foot-high true arch dam is constructed in the Libby Formation of the Precambrian Belt Supergroup. This dam in coordination with the Albeni Falls dam downstream controls the water level of Lake Pend Oreille, normally 2062 feet. The prominent terrace south of the river is mostly Missoula flood deposits but logs from monitor wells drilled in 1952 by WWP show cycles of clay till and interbedded lake deposits indicating multiple episodes of ice damming (Harold T. Stearns, oral communication, 1986). Glacial erosion as well as till deposits indicative of an ice margin are found in this area, so many geologists have shown the ice lobe terminus near here. The bedrock bench on the north side of the valley has abundant till cover, interpreted to be late glacial ice- marginal deposits; possibly flood drainage was diverted to the south side of the valley by the ice from the north, at least in the waning stages of smaller late floods. The bench may represent the edge of the ice dam failure or the margin of subglacial flow. Just to the north, ice flowed through cols as high as 6000 feet in elevation across the Cabinet Mountains between the Bull River and the Purcell Trench. The ice lobe in the Bull River drainage is not thought to have reached the Clark Fork valley.

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Glacial Lake Missoula was determined by Pardee to contain 500 cubic miles of water with a surface area of 2900 square miles and a depth of 2000 feet (Figure 15). These figures were based on the elevation of the highest evidence for a lake level from evidence of old shorelines. He determined the highest shorelines on the mountain slopes in the Missoula valley at an elevation of at least 4200 feet.

Figure 15. Colorized version of figure by J.T. Pardee, 1910, Figure 4 page 382.

Eddy Narrows

Pardee (1942) calculated the volume of water that he believed moved through Eddy Narrows during the largest of the flood events at 9.5 mi3 (39.9 km3) of water per hour; this at a flow rate of ~44 mi/hr (71 km/hr). He also noted that the fast-moving water had stripped the surrounding walls of much of their topsoil and talus up to the level he estimated to be the highest stand of the lake (Pardee, 1942). Old 1933 photographs of the lower Clark Fork valley reveal spectacular shorelines along the valley directly south of Cabinet Gorge perhaps as high as 4,260 feet (Figure 16). Depending on the lighting careful observers can see the shorelines across the valley. Putting these observations together suggests there have been scores of Missoula Floods and Lakes Missoula with a wide range of volume and discharge (Figure 17).

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Figure 16. Washington Air Guard photo overlaid on topography in Google Earth showing glacial Lake Missoula strandlines near Dry Creek.

Figure 17. 1. Ice advances or retreats up and down the PT and fills the LPO basin.2. As the ice thickens a tongue spills into the Clark Fork valley damming the river. 3. Ice advances up the valley forming a shallow but deepening lake. 4. As the lake fills the terminus is eroded by calving buoyant ice and ablation. 5. The cycle reaches a penultimate stage. The maximum level of the lake is restrained by a thick but short plug precariously near the mouth of the valley and the lake basin. This progression would have led to larger and deeper lakes and floods until such time as the glacier started to diminish in response to climate change. The late lakes and floods may have been quite small.

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Proglacial deposits

A feature deposited at the mouth of Dry Creek with the Clark Fork valley is well exposed in a gravel pit. The pit is at an elevation of 2680 feet, over 400 feet above the present floor of the valley. The foresets are tens of feet high and dip up river to the east. The clasts are poorly rounded, and some cobbles are striated. Although most of the clasts are of Precambrian Belt metasediments derived locally, some are from granite and diorite (Purcell sills) from rocks exposed in the Purcell Trench. The granules are cemented, and the pit walls must be ripped before the aggregate can be excavated. This gravel probably was deposited in latest-glacial time from the ice margin into Lake Missoula in the form of a pro-glacial delta (Gilbert type). It is not to be confused with the so-called gulch fills or eddy bars of Pardee that formed in the side-valleys during Missoula Floods. Most of the drainages on the south side of the Clark Fork valley from Lake Pend Oreille upstream to nearly Thompson Falls contain these features. Thus late glacial ice advanced upstream much farther than had been previously recognized. Furthermore, late phases of glacial Lake Missoula associated with this ice dam drained quiesently enough to leave the deposits preserved.

Because the proglacial deltas and kame terraces in the Clark Fork valley left by this advance are intact they must post-date the major catastrophic Missoula floods. Furthermore, , expansion bars and other flood deposits in the lower Clark Fork valley are mantled by lacustrine silts indicating that the last glacial Lake Missoula did not drain catastrophically (Figure 18). Waitt (1985) interprets 14C evidence to show that glacial Lake Missoula existed only between about 17,200 and 11,000 years ago. The Purcell Trench was free of Cordilleran ice by the time of two closely spaced Glacier Peak eruptions about 11,200 years ago (Carrara et al. 1996). Kame deposits near the United States- Canada border constrain the ice recession (10Be range from 13.3 to 7.7 ka (W. Phillips, written communication, 2011). Latest glacial alpine moraines formed by valley glaciers in the Selkirk and Cabinet Ranges occupy the tributary valleys of the Purcell Trench. These deposits must post-date retreat of the Cordilleran ice lobe and are evidence for a late Wisconsin alpine glacial advance. An uncalibrated radiocarbon age of 9,510 ± 110 BP (Mack and others, 1978) from a peat bog within the glacial limit on the west slope of the Selkirk Mountains is a minimum -limiting age on the last alpine ice. The bus tour will backtrack to Clark Fork on Hwy 200 to the Clark Fork Bridge Stop.

CLARK FORK BRIDGE STOP

DT journal entry for October 11 1809 traveling upstream on the Clark Fork River:

Weds Oct 11 A fine Night & fine Day – early looked for the strayed Horse but could not find him Sent for another which being brought we got ready & at 10 ½ AM set off

Came to the Indian Tents Co[urse] by the Compass S 81 E 2M[iles] sent the young man across for his Father who is to be our Guide—our Co will be S 66 E

at ½ PM set off & by 2PM we were on top of the River Hills hav[i]n[g] crossed 2 Brooks from hence we see the House Point Clearly

Crows calling we sent the young Man to see what it was who returned at 4 ½ PM with a good Cord of fat Chevruil which he took from the

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The geotechnical information for the foundation design of the new bridge encounters bedrock at 615 feet on the north end and no bedrock at 617 feet on the south end. The cores penetrated surface silts underlain by coarse gravel in turn underlain by fine sand and silt indicating alternating high and low energy regimes.

Figure 18. Geologic map of the Clark Fork area compiled from Idaho Geological Survey Digital Web Maps 24, 25, 59, and 60. Red dot indicates Clark Fork Bridge field stop.

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SELECTED REFERENCES

Alden, W.C., 1953, Physiography and glacial geology Breckenridge, R.M., and Lewis, R.S., 2005, Field Trip of western Montana and adjacent areas: U.S. to the Clark Fork ice dam and Missoula Floods Geological Survey Professional Paper 231, 200 p. outburst area, Idaho: Ice Age Floods Institute, 17 p.

Baker, V.R., 1973, Paleohydrology and Breckenridge, R.M., and Phillips, W.M., 2010, New of Lake Missoula flooding in eastern Washington. cosmogenic 10Be surface exposure ages for the Purcell Geological Society of America Special Paper 144, 79 Trench lobe of the Cordilleran ice sheet in Idaho: p. Geological Society of America Abstracts with Programs, v. 42, n. 5, p. 309. Baker, V. R., and Nummedal, D., eds., 1978, The Channeled Scabland: National Aeronautics and Space Breckenridge, R. M., and Othberg K.L., 1994, Timing Administration, 186 p. of glacial Lake Missoula floods - Paleomagnetic evidence from glacial lake sediments near Priest Baker, V.R., ed., 1981, Catastrophic Flooding— The River, Idaho: Geological Society of America, origin of the Channeled Scabland, Benchmark Papers Abstracts with Programs, v. 26, n. 7, p. A-307. in Geology v. 55, Dowden, Hutchinson, and Ross, Inc., 360 p. Breckenridge, R.M., and Othberg, K.L., 1998, Chronology of glacial Lake Missoula floods- Breckenridge, R.M., 1989a, Evidence for ice dams Paleomagnetic evidence from Pleistocene lake and floods in the Purcell Trench: Trip 1A, in N.A. sediments near Clark Fork, Idaho: Geological Society Joseph and others, eds., Geologic Guidebook to of America Abstracts with Programs, v. 30, n. 6. p. 5. Washington and Adjacent Areas, Washington Division of Geology and Earth Resources, Breckenridge, R.M., and Sprenke, K.F., 1997, An Information Circular 86, p. 309-320. overdeepened glaciated basin, Lake Pend Oreille, northern Idaho: Glacial Geology and Geomorphology, Breckenridge, R.M., 1989b, Pleistocene ice dams and RP03, John Wiley, 26 p. glacial Lake Missoula Floods in northern Idaho and adjacent areas, in V.E. Chamberlain, R.M. Bretz, J H., 1923, The Lake Missoula Floods and the Breckenridge, and Bill Bonnichsen, eds. , Guidebook Channeled Scabland, Journal of Geology, v. 77, p. to the Geology of Northern and Western Idaho and 505-543. Surrounding area. Idaho Geological Survey Bulletin 28: 5-16. Bretz, J H., 1932, The Channeled Scabland: 16th International Geological Congress U.S. Guidebook Breckenridge, R.M., 1989c, Lower glacial lakes Excursion C-2, 16 p. Missoula and Clark Fork ice dams, in Breckenridge, R.M., ed., Glacial Lake Missoula and the Channeled Bretz, J H., Smith, H.T.U., and Neff, G.E., 1956, Scabland. American Geophysical Union, 28th Channeled Scabland of Washington: New data and International Geological Congress, Field Trip interpretations: Geological Society of America Guidebook T310, p. 13-23. Bulletin, 67, p. 957-1049.

Breckenridge, R.M., 1991, Ice-dam margins in lower Carrara, P.E., Kiver, E.P., and Stradling, D.F., 1996, Clark Fork valley, Idaho and Montana. Abstract The southern limit of Cordilleran ice in the Colville Volume with Meeting Program, 38th Pacific and Pend Oreille valleys of northeastern Washington Northwest American Geophysical Union Meeting, p during the Late Wisconsin glaciation: Canadian 11. Journal of Earth Sciences, v. 33, p. 769-778.

Breckenridge, R.M., and Garwood, D.L., 2012, Late Clague, J.J., 1986, The Quaternary stratigraphic Wisconsin glaciation in northern Idaho: Annual record of British Columbia— evidence of episodic meeting of the Pacific Northwest Section of the sedimentation and erosion controlled by glaciation: National Association of Geoscience Teachers, Canadian Journal of Earth Sciences, v. 23, p. 885-894. Spokane, Washington. Clague, J.J., Armstrong, J.E., and Mathews, W.H., 1980, Advance of the late Wisconsin Cordilleran ice

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sheet in southern British Columbia since 22,000 yr. O’Connor, J.E., and Baker, V.R. 1987, Peak flood BP: Quaternary Research, v. 13, p. 322-326. discharges from late Pleistocene glacial Lake Missoula. Geological Society of America, Abstracts Desloges, J.R., and Gilbert, Robert, 1991, with programs, v. 19, p. 792. Sedimentary record of : implications for deglaciation in southwestern British Columbia: Pardee, J.T., 1910, The glacial Lake Missoula: Journal Canadian Journal of Earth Sciences, v. 28, p. 800-815. of Geology, v. 18, p. 376-386.

Elliott, T.C., 1920, David Thompson’s journeys in Pardee, J.T., 1942, Unusual currents in glacial Lake Idaho: Washington Historical Quarterly 11, p. 97-103. Missoula, Montana: Geological Society of America Bulletin, v. 53, p. 1569-1600. Eyles, N., Mullins, H.T., and Hine, H.C., 1990, Thick and fast: Sedimentation in a Pleistocene fiord lake of Richmond, G.R., 1986, Tentative correlation of British Columbia, Canada: Geology, v. 18, p. 1153- deposits of the Cordilleran ice-sheet in the northern 1157. Rocky Mountains, in G.M. Richmond and D.S. Fullerton, eds. Quaternary Glaciations in the United Fulton, R.J., and Smith, G.W., 1978, Late Pleistocene States of America: Quaternary Science Reviews, 5, stratigraphy of south-central British Columbia. Quaternary Glaciations in the Northern Hemisphere, Canadian Journal of Earth Sciences, v. 15, p. 971-980. p. 99-128.

Gilbert, Robert, 1975, Sedimentation in Lillooet Lake, Shaw, J., Munro-Stasuik, M., Sawyer, B., Beaney, C., British Columbia: Canadian Journal of Earth Sciences, Lesemann, J.E., Musacchio, A., Rains, B., and Young, v. 12, p. 1697-1711. R.R, 1999, The Channeled Scabland—Back to Bretz?: Geology, v. 27, p. 605-608. Gilbert, Robert, and Desloges, J.R., 1992, The late Quaternary sedimentary record of Stave Lake, Smyers, N. B., and Breckenridge, R.M., 2003, Glacial southwestern British Columbia: Canadian Journal of Lake Missoula, Clark Fork ice-dam and the floods Earth Sciences, v. 29, p. 1997-2006. outburst area: Northern Idaho and western Montana, in Swanson, T.W., ed, Western Cordillera and Mack, R.N., Rutter N.W., Bryant Jr., V.M., and adjacent areas: Geological Society of America Field Valastro, S., 1978, Re-examination of postglacial Guide-4, p. 1-17. vegetation history in northern Idaho: Hager Pond, Bonner County. Quaternary Research, v. 10, p. 241- Waitt, R.B., 1984, Periodic jökulhlaups, from 255. Pleistocene glacial Lake Missoula—New evidence from varved sediments in northern Idaho and Meyer, S.E., Gaylord, D.R., Foit, F.F., and Washington: Quaternary Research, v. 24, p. 357-360. Breckenridge, R.M., 1999, Pre-late Wisconsin glacial Lake Missoula flood deposits in the Latah Creek Waitt, R.B., 1985, Case for periodic, colossal valley, Washington and other proximal flood deposits, jökulhlaups from Pleistocene glacial Lake Missoula: northern Washington and Idaho: Geological Society of Geological Society of America Bulletin, v. 96, p. America Abstracts with Programs, v. 31, n.6, p. A-79. 1271-1286.

Mullins, H.T., Eyles, N., and Hinchey, E.J., 1990, Waitt, R.B., Denlinger, R.P., and O’Connor, J.E., Seismic reflection investigation of Kalamalka Lake a 2009, Many monstrous Missoula floods down "fiord lake" on the Interior Plateau of southern British Channeled Scabland and Columbia Valley, in Columbia: Canadian Journal of Earth Sciences, v. 27, O’Connor, J. E., Dorsey, R.J., and Madin, I. P., eds., p. 1225-1235. Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest: Nisbet, Jack, 2007, Sources of the River, Tracking Geological Society of America Field Guide 15, p. David Thompson across Western North America, 2nd 775-844. edition, Sasquatch Books, 304 p. Walker, E.H., 1967, Varved lake beds in northern Nisbet, Jack, 2005, Map Makers’ Eye, David Idaho and northeastern Washington: U.S. Geological Thompson on the Columbia Plateau, Washington Survey Professional Paper 575B, p. B83-B87. State University Press, 232 p.

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