Quaternary Diversion and Incision, Dearborn River, Montana: Summary

Quaternary diversion and incision, Dearborn River, Montana: Summary

MICHAEL G. FOLEY"' Department of Geology, University of Missouri-Columbia, Columbia, Missouri 65211

INTRODUCTION

Pinedale stades (after Birkeland and others, 1971) Direct measurements of stream-incision rates are generally neither practical nor meaningful because bed-rock abrasion rates Upper Middle Lower are so small, and variability of average stream discharge and sediment load so large. Field occurrences of bed-rock incision by- streams rarely allow determination of incision duration, initial channel geometry, or history of flow parameters. However, incision resulting from datable stream diversion or capture has a known duration. In the example reported herein, a glacially diverted river has cut a new course up to 60 m into bed rock since diversion. Initial geometry, maximum discharge, and sediment transport rate can be estimated for the diverted channel, in addition to the time of diversion. This is a natural, long-term incision experiment for evaluating a deterministic model of bed-load abrasion and stream incision 10,000 20,000 (Foley, 1980a, 1980b). Years B P. INCISION CHRONOLOGY Figure 2. Incision chronology for the middle of the Dearborn River diverted channel. The Dearborn River, a tributary of the Missouri River in Lewis and Clark and Cascade Counties, northwestern Montana (Fig. 1), correlating them with the glacial moraines. This reconstruction was drains —1,420 km2 of the Lewis and Clark Range and adjacent based in part on relations between terraces and nickpoints on the high plains. Two end moraines lying athwart the former course of former and present river and its tributaries. Terraces were scarce the Dearborn River outline the piedmont lobe(s) of the Pinedale age along the diverted channel and in general defied detailed correla- (Richmond, 1965) Dearborn Glacier which diverted the river to its tion in the rest of the basin. Accelerated incision following glacial present course. The beheaded course is now occupied by Flat periods, or extensive outwash emplacement during them, has not Creek, a small underfit stream. produced extensive, easily correlated terraces along the main stem. A time-elevation history of incision in the diverted reach was es- Instead, nickpoints and terraces on tributaries record periods of tablished by reconstructing former valley longitudinal profiles and stability followed by accelerated incision, and they help establish valley elevation at the confluences immediately prior to those events. Two former valley levels can be identified on the present course of the Dearborn River. Some terraces on the main stem correlate with these levels as do extensive terraces along some of the tributaries. These levels were extended through the diverted reach by projecting the basal surface of glacial deposits on the canyon walls above the diverted reach across the canyon. Resulting estimated valley-bottom positions are maximum elevations, because ice-gouging was ignored. However, ice motion was normal to the former valley, and so this effect is probably minor. The most likely combination of events is that the Dearborn River was completely diverted across a former divide by a lower Pinedale glacial lobe. Incision continued in the diverted channel during the ensuing interstadial and was accelerated during the middle Pinedale Figure 1. Index map of the Dearborn River study area, stade when the inner moraine was deposited. Incision continued to Montana. the present, with the lowest rate (0.5 mm yr~') at present, inter- mediate rates during irterstadials and upper Pinedale stade, and the ''Present address: Battelle Pacific Northwest Laboratories, P.O. Box 999, highest rate (7 mm yr~') during the lower and middle Pinedale Richland, Washington 99352. stades. This reconstruction is shown in Figure 2.

The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 91, no. 10, p. 2152-2188.

Geological Society of America Bulletin, Part I, v. 91, p. 576-577, 2 figs., October 1980, Doc. no. S01002.

576

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PALEOHYDRAULICS AND BED-ROCK ABRASION constructed former diverted channels were calculated using the

The time span of Figure 2 is sufficiently long that short-term per- Manning and Meyer-Peter and Miiller equations, respectively. turbations may be ignored, and a mathematical model of stream incision may be compared with a meaningful average behavior. In ACKNOWLEDGMENTS the reconstruction of events, the channel longitudinal profile is This work was funded by a grant from the Research Council of known at two times during incision. If channel cross-sectional the Graduate School, University of Missouri-Columbia. shape is assumed to be similar to that of the modern channel, then hydraulics and sediment transport rate during incision may be es- REFERENCES CITED timated if discharge and sediment characteristics can be deter- mined. Birkeland, P. W., Crandell, D. R., and Richmond, G. M., 1971, Status of The abandoned Dearborn River channel now occupied by Flat correlation of Quaternary stratigraphie units in the western conter- minous United States: Quaternary Research, v. 1, p. 208-227. Creek has been little modified since it was carrying outwash from Foley, M. G., 1980a, Bed-rock incision by streams: Geological Society of the outer moraine glacier. Maximum outwash discharge in the Flat America Bulletin, Part II, v. 91, no. 10, p. 2189-2213. Creek valley was about 3.3 X 103 m3s~', based on flow depth re- 1980b, Bed-rock incision by streams: Summary: Geological Society of quired to move the largest outwash boulders, and the simple Man- America Bulletin, Part I, v. 91, p. 577-578 (this issue). ning equation. Maximum outwash discharges in the diverted chan- Richmond, G. M., 1965, Glacial moraines and outwash terraces on the 2 3 _1 2 3 _1 plains east of the Central Montana Ice Cap: International Quaternary nel were 7 x 10 m s and 6 X 10 m s during outer moraine Association Congress, 7th, Boulder, Field Conference E Guidebook: and inner moraine glaciations, respectively, based on ratios of Lincoln, Nebraska, Nebraska Academy of Sciences, p. 55. glacier areas within the present drainage divides related to each to those of the Flat Creek outwash channel. Outwash gravel in the diverted channel was assumed to be similar to that in the Flat Creek valley outwash (mean size 1.25 cm), and paleohydraulics and MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 23, 1979 REVISED MANUSCRIPT RECEIVED APRIL 21, 1980 sediment-transport rates for maximum flood flows in the two re- MANUSCRIPT ACCEPTED MAY 7, 1980

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