Must See Floods Features in Northern Idaho

SOME LOCAL NORTHERN IDAHO ICE AGE FLOODS and GLACIAL FEATURES Purcell Trench – This structurally formed valley became the passageway for the Purcell Ice Lobe that divided into five glacial sub-lobes. The east moving sub-lobe blocked the Clark Fork River forming Glacial Lake Missoula, the source of the Ice Age Flood waters. Other sub-lobes modified and deepened the basin now occupied by Lake Pend Oreille or helped define several important N-S transportation routes. The Purcell Trench is visible from City Beach in Sandpoint and Schweitzer Mountain Rd. Hwy 95 follows the Purcell Trench from Sandpoint to beyond Bonners Ferry. Cabinet Gorge – Contact of Pleistocene Ice and Glacial Lake Missoula Water – Although the contact of the Pleistocene Ice and the lapping waters of Glacial Lake Missoula was in flux as the glacier advanced and retreated, the Cabinet Gorge is considered by many to be a likely location of the terminus of the Clark Fork sub-lobe that blocked the drainage of the Clark Fork River. Evidence of glacial erosion and till deposits indicate glaciation; however, flood deposits on the south side of the river attest to Ice Age flooding. Cores taken during the construction of the Cabinet Gorge Dam suggest multiple stages and events of ice damming. An AVISTA maintained viewpoint at the Cabinet Gorge Dam with Ice Age Floods signage provides an excellent location to ponder the formation and disintegration of the ice plug blocking the Clark Fork River. Glacial Striations – Striations can be viewed along Hwy 200 east of Hope and on the south side of Castle Rock east of Clark Fork. Glacial/Flood Related Erratics – Large and small erratics cover the glacial and flood modified landscape. Large erratics can be seen along Hwy 200 east of Clark Fork, in Sandpoint’s City Beach and in Farragut State Park. A large pile of erratics excavated from flood deposits are visible on the west-side of Hwy 95 south of the Hoodoo Channel. A giant ice-rafted flood related erratic weighing over 1,600 tons is located on the east-side of Hwy 41 near Twin Lakes. Lake Pend Oreille – Cross-section morphology, depth, and sediments – This is where it all started! Lake Pend Oreille is the largest lake in Idaho and the deepest lake in the Pacific Northwest and the 5th deepest in the US. The lake level is 2062 ft above sea level with depths over 1,000 feet and an additional layer of over 1,500 feet of glacial-fluvial deposits going to bedrock, with the surrounding terrain as high as 6002 ft. The glacially modified U-shaped bedrock valley, cut to approximately 600 feet below sea level, was formed by the Pend Oreille glacial sub-lobe. This glacial sub-lobe was up to 4,000+ ft. thick and 30+ miles wide. It was the ice plug responsible for blocking the Clark Fork River. It collapsed catastrophically as often as about every 10-60 years, releasing 40-100 Ice Age Floods. The location of the lake is probably related to an old river valley controlled by faults. The Lake Pend Oreille basin was carved by the repeated advances of Pleistocene ice and scoured by ice age floods. With the waning of ice age flood waters, the basin was, and continues to be, filled with glacial outwash and flood deposits. The lake is dammed at the south end by thick glacial and flood deposits the mark the beginning of the “Outburst Deposits”. There is a pullout on Hwy 200 approximately one mile west of Hope with signage already established that provides a great view looking south across Lake Pend Oreille to the Green Monarch Mts. Another view of the former ice plug location along Hwy 200 is the mouth of the Clark Fork River at the Clark Fork Drift Yard. Giant Current Dunes or MegaRipple Marks – One of Bretz’ most important pieces of evidence for catastrophic flooding was the “giant current dunes.” These large-scale bedforms appeared as patterns of parallel ridges and swales on many aerial photographs in the flood channels in the scabland of Washington, but had escaped recognition from the ground because of their size. Giant Current Dunes are visible west of Clark Fork near Castle Rock; however, the most prominent and visible Giant Current Dunes are located several miles east of Spirit Lake along Hwy 54 where the highway cuts through the dunes showing their undulating profile. The location of many of the telephone poles on the crest of these dunes accentuates these landforms. The dunes form transverse to the current direction, and form cusps that are convex upstream, with arms that point downstream. Furthermore, the size of the cusps appears to decrease in the direction of lower velocity. Internally, the dunes consist of gravel and pebble foresets. Giant current dunes exhibit an asymmetrical profile with the downstream (lee) slope steeper than the upstream slope. Crests range from 20 to 200m apart and heights range from 1 to 15m (Baker and Nummedal, 1978) and are among the largest measured throughout the Floods area. The Spirit Lake current dunes can also be easily recognized from the air by their characteristic pattern, accentuated by vegetation. This dune field is immediately in the path of the breakout from Lake Pend Oreille, and experienced some of the highest energy flows. Rhythmites – Rhythmites are glaciolacustrine sediments associated with flood events and are useful in attempting to determine the number of Ice Age Floods. A 112-foot cross-section of rhythmites is located at the junction of East Fork Creek and Lightning Creek approximately 6.5 miles northeast of Clark Fork along Lightning Creek Road (NF-419). Proglacial Deposits – This type of deposit is found associated with most of the drainages on the south side of the Clark Fork Valley and extend from the mouth of the Clark Fork River to nearly Thompson Falls in Montana. These sediments are glacio-fluvial deposits deposited in contact with the ice and therefore provide evidence of glaciation in the Clark Fork River drainage. The associated geomorphic features formed by these deposits are referred to as “kame deltas”. The gravel pit associated with Dry Creek (approximately 8 miles west of Clark Fork
Museum of North Idaho

Museum of North Idaho IceAgeFloods National GeologicTrail “The Museum of North Idaho collects, preserves and interprets the history of the Coeur d’Alene Region. to foster appreciation of the area’s heritage.” In the 1960s, the North Idaho Hoo Hoo Club, an organization of loggers, lumbermen and foresters, first entertained the idea of establishing a museum. They incorporated on May 1, 1968 for a museum focusing on the history of the Coeur d’Alene Region (Kootenai, Benewah and part of Shoshone counties). Through the efforts of volunteers and community support, the Museum opened its doors on the North Idaho College campus on July 28, 1973. In 1979, they remodeled a City-owned building and relocated to its current location. Quick Facts Location: 115 Northwest Blvd, Coeur d’Alene, ID 83814Phone: (208) 664-3448 The Museum is open Tuesday to Saturday from 11 am – 5 pm.
Lake Pend Oreille

Lake Pend Oreille Ice Age Floods National Geologic Trail The Purcell Lobe of the Cordilleran Ice Sheet repeatedly formed a 2000′ tall ice dam in this area, which backed up Glacial Lake Missoula and eventually burst catastrophically, releasing Ice Age Floods as often as about every ~10-60 years. Lake Pend Oreille is the largest lake in Idaho. The lake level is 2062 ft above sea level, with the surrounding terrain as high as 6002 ft. With a maximum depth of 1150 ft, this lake is the 5th deepest in the US. The location of the lake is probably related to an old river valley controlled by faults. The Lake Pend Oreille basin was carved by the repeated advances of Pleistocene ice and scoured by Ice Age Floods. With the waning of flood waters, the basin was filled with glacial out-wash and flood deposits. The lake is dammed at the south end by thick glacial and flood deposits that mark the beginning of the “Outburst Deposits”Quick Facts MANAGED BY:US Forest Service
Green Monarch Ridge

Green Monarch Ridge Ice Age Floods National Geologic Trail THIS IS WHERE IT ALL BEGAN! The Green Monarch Ridge viewpoint, just south of the Hope Fault, is positioned along the edge of the Purcell Trench Ice Lobe. Here it was responsible for damming the mouth of the Clark Fork drainage basin. This damming effect resulted in the formation of Glacial Lake Missoula, the source of the waters for much of the PNW Pleistocene Ice Age Floods. The Purcell Trench Ice Lobe originated in Canada and flowed south into Idaho, guided by the structural control of the Purcell Trench. Following the path of least resistance into the basin now occupied by Lake Pend Oreille, it was impeded by the Green Monarch Ridge, located approximately 10 miles south of this viewpoint. Grinding and scraping, the Purcell Trench Ice Lobe backed up behind the Green Monarch Ridge, eventually building an ice plug up to 4,000 feet thick, and scouring out the deepest parts of Lake Pend Oreille. The Purcell Trench Ice Lobe divided into four sub-lobes: one went approximately west (US Highway 2), two went south (US Highway 95 and Lake Pend Oreille) and one went east (SR 200). All four sub-lobes contributed to blocking the Clark Fork drainageQuick Facts MANAGED BY:US Forest Service – Idaho Panhandle National Forest
Glacial Dam at Green Monarch Ridge View Point

Glacial Lobe Dam at Green Monarch Ridge View Point Ice Age Floods National Geologic Trail As the Purcell Trench ice lobe of the Cordilleran Ice Sheet advanced south from Canada, it was stopped by the Green Monarch Ridge, building an ice dam 4,000 feet (1219 m) tall and nearly 40 miles (64 km) wide that blocked the Clark Fork river, thus filling glacial Lake Missoula. View the Green Monarch Ridge and the Purcell Trench from a large pullout on Idaho State Route 200, about one mile (1.6 km) west of Hope, Idaho and 15 miles (24 km) east of Sandpoint, Idaho. Quick Facts Location: Idaho State Route 200, about one mile west of Hope, Idaho and 15 miles east of Sandpoint, Idaho.
Farragut State Park

Farragut State Park Ice Age Floods National Geologic Trail Farragut State Park is located at the “breakout” of Glacial Lake Missoula floods, where the ice dam in the Clark Fork valley and the 20-mile-long tongue of ice occupying the Lake Pend Oreille basin failed. From there, a torrent of water and ice burst from the south end of the lake. Farragut State Park is located at the “breakout” of Glacial Lake Missoula floods. Failure of the ice dam in the Clark Fork valley fractured and broke apart the 20-mile-long tongue of ice occupying the Lake Pend Oreille basin, and a torrent of water and ice burst from the south end of the lake. That churning slurry of flood waters, ice, boulders and other debris erupted from the south end of Lake Pend Oreille, flowing 2000 ft deep across Farragut State Park. Nearly all of the water escaping from Glacial Lake Missoula passed through this area. Discharge estimates range from 14 to 21 million cubic meters per second! Vic Baker’s definition of a megaflood is one which has a discharge of at least 1mil cubic m/s, so we are possibly looking at ground zero for the worst Pleistocene flood in the world. Farragut State Park includes many geologic features left by glaciation and megafoods.Hoodoo channel, an abandoned outlet of Lake Pend Oreille provided a pathway for late-glacial meltwater and for the last outbursts from Glacial Lake Missoula. The channel is marked with a number of closed depressions, probably the result of melting icebergs. One of the largest of these features have been proposed, including kettles, potholes, and sub-glacial meltwater. Some of that debris was deposited to form the Rathdrum Prairie Outburst Plain, an extensive, heterogeneous mix of flood deposits. Farragut State Park offers unique scenery, history and an abundance of recreational opportunities. Quick Facts Location:13550 E. Hwy 54 Athol, ID 83801
Clark Fork Ice Dam

Ice Age Floods National Geologic Trail 12,000 to 17,000 years ago a 4000′ tall ice dam blocked the path of the Clark Fork River creating glacial Lake Missoula. At the end of the last ice age this ice dam failed, releasing more water than is held in modern day Lake Ontario and Lake Erie combined. 600 cubic miles of water rushed through this area and down Lake Pend Oreille destined for the Pacific Ocrean.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 foods. 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. Quick Facts Location:Clark Fork, Idaho