The Lower Chippewa Valley, Wisconsin

GEORGE \\. ANDREWS

LATE QUATERNARY GEOLOGIC HISTORY OF THE LOWER CHIPPEWA VALLEY, WISCONSIN

Abstract: The lower Chippewa Valley in west- Valley are closely linked to the post-Gary history central Wisconsin extends 65 miles from the Gary of the lower Chippewa Valley, for these factors terminal moraine in Chippewa County to the controlled the outlet level of the Chippewa River. Mississippi River Valley. The Chippewa Valley and This outlet was substantially lower than at present its tributaries were filled with a valley train of sand throughout much of post-Gary Pleistocene and and gravel during the maximum stand of the Gary early Recent time. The modern Chippewa River ice, and entrenchment of this deposit has formed has built a delta into the Mississippi Valley. The the Wissota terrace, a prominent geomorphic Chippewa River is aggrading the lower part of its feature that can be traced the length of the valley. valley, a meandering river is slowly eroding the Several lower terraces in the valley indicate pro- central part; stream erosion in the upper part is gressive downcutting of the Wissota terrace sedi- restricted by sills of hard bedrock. ments. Erosion and deposition in the Mississippi

308-311) recognized the glaciofluvial origin of Introduction the deposits and presented a reconnaissance The Chippewa River has its source on the map of the terraces in the portion of the valley Gary ground moraine in northern Wisconsin, between the Gary terminal moraine and the crosses the Gary terminal moraine near Jim mouth of the Red Cedar River (PI. 28). The Falls, Chippewa County, Wisconsin, and veers "... series of terrace levels carved in the out- southwest across older drift along the northern wash gravels" near Eau Claire, Wisconsin, was margin of the Driftless area. Terraces are noted by Martin (1932, p. 360), but he did not prominent topographic features of the lower comment further on their origin or extent. Chippewa Valley, which extends from the Figure 1 is an index map of the study area. Gary terminal moraine to the confluence of the Field observations have been made in the Chippewa and Mississippi rivers at the south area over several years, particularly during the end of Lake Pepin. The tributary valleys of summers of 1957 and 1960. Laboratory studies this region have physiographic unity with the were conducted with the aid of U. S. Geological major valley, for their depositional and ero- Survey topographic maps, aerial photographs sional history has been closely controlled by of the Wisconsin State Highway Commission, that of the major valley. Although the terraces and aerial photograph index sheets of the of the lower Chippewa Valley have been noted Commodity Stabilization Service, U. S. De- by observers for more than a century, no partment of Agriculture. systematic examination of their nature or extent has been made. This study discusses the Wissota Terrace geomorphic features of the area and the late The highest recognizable terrace in the lower Quaternary regional history. Chippewa Valley is herein termed the Wissota Very little has been published on the terraces terrace because of its excellent development of the lower Chippewa Valley. The earliest around the shores of Lake Wissota, a reservoir observations were made by Owen (1848, p. on the Chippewa River, Chippewa County. 17-18, PI. 8), who recognized the alluvial The Wissota terrace is the highest and the nature of the terrace deposits and postulated most prominent and extensive terrace in the their derivation from the friable Cambrian valley. Although it cannot be followed con- sandstones of the area. Wooster (1882, p. 137- tinuously on either side of the valley because of 138) recognized the "fluvial or lacustrine" incision by tributary streams, the terrace level nature of the terrace deposits and also noted can be traced without difficulty throughout the the multiple terrace levels in parts of the 65-mile length of the lower Chippewa Valley. valley. Chamberlin and Salisbury (1885, p. The Chippewa Valley behind the Gary termi-

Geological Society of America Bulletin, v. 76, p. 113-124, 5 figs., 1 pi., January 1965 113

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nal moraine does not seem to have a correspond- tary valleys and are all considered part of the ing terrace level. This study has not been Wissota terrace (Fig. 2). extended into the Mississippi Valley because The Wissota terrace slopes from about 950 the correlative high terrace in that valley, if it feet near Jim Falls to about 770 feet near the

I rf ; Menomome > MQund. loHiPPEWA OQ.JL.

DUWJ CO.U jjT ^.. PEPIN" co. VST c.Jr

I_EALI_CLAIRE_ CO.- I [MINNESOTA /-- ~ - Mondovi I

''•^ /JLUXOlSl" | Figure 1. Index map of the lower Chippewa Valley region, west-central Wisconsin

ever existed, seems to have been removed or at mouth of the river at the lower end of Lake least modified by subsequent erosion. Pepin. A profile of the slope (Fig. 3) shows The Wissota terrace is not restricted to the some minor irregularities resulting from initial main Chippewa Valley; comparable terrace deposition, but in general the gradient is levels are found in all major and minor tribu- remarkably constant and averages 3.16 feet

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per mile. For the lower 7 miles of the valley, Valley in the lowermost part of the Chippewa however, the terrace remnants preserved on Valley. the left bank drop gradually to an elevation of The Wissota terrace-margin at the valley about 730 feet. This terrace was lowered by wall is often difficult to determine precisely,

[ JWissota terrace remnants Wissota terrace deposits n removed by erosion.

Figure 2. Geographic extent of the Wissota terrace and inferred original extent of the terrace deposits, west-central Wisconsin

later erosion. The levelling of the terrace on especially in the tributary valleys. This contact the right bank at 770 feet along the lower 7 was initially a fairly sharp break in slope, but miles of the valley is possibly explained by the it has been modified by subsequent deposition deposition of excess debris from the Mississippi of locally derived sediment at many places. The

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terrace surface has been modified by wind erosion and deposition as well as by slopewash and stream erosion and deposition. The sandy nature of the terrace deposits permitted the formation of blowouts and sand dunes before a vegetative cover was firmly established. Most sand dunes are now fixed, but mobile sand can be observed at places. The initial depositional surface of the Wissota terrace seems to have been remarkably smooth, and relief greater than 5 or 10 feet in a given area is rare. As the terrace approaches the valley wall it slopes gently upward, with the greatest slope near the valley wall. The more central portions of the terrace show little, if any, recognizable lateral curvature. A system of braided ridges and swales of low relief parallel the length of the main valley near its center (PI. 1, fig. 1), whereas closer to the margins of wider sections of the valley and extending into the tributary valleys the pattern of ridges and swales has no preferred orientation. No meandering pattern has been found on the Wissota terrace tread. The Wissota terrace deposit is predominately a pebbly coarse sand; pebbles and cobbles are common, boulders rare. Although the deposit contains some fine sand and interstitial silt and clay, no argillaceous beds occur. The pebbles consist principally of well-rounded to sub- rounded igneous and metamorphic rocks, mostly very hard and apparently not weath- ered. This type of deposit is commonly referred to as a gravel, and it will be so regarded in this paper. It should be emphasized, however, that the dominant constituent of the Wissota terrace gravel is sand rather than pebbles. The Wissota terrace sediment shows well-developed but discontinuous bedding and cross-bedding in fresh cuts. Cross-bedded layers range from a few inches to more than 1 foot in thickness. Precambrian granite or gneiss probably con- tinuously underlie the Wissota terrace gravel from the Gary terminal moraine to below Chippewa Falls (see Fig. 3). The Mount Simon Sandstone and possibly other Upper Cambrian formations of younger age occur downstream. Records of wells at Eau Claire, sees. 5 and 19, T. 27 N., R. 9 W., suggest a bedrock elevation of about 700 feet and a thickness of about 185 feet of Wissota terrace gravel. A test well south

of Eau Claire (sec. 30, T. 27 N., R. 9 W.) sug- 133d Nl NOI1W313 gests a bedrock elevation of about 615 feet and a thickness of 260 feet of Wissota terrace gravel. Bedrock depth and thickness downstream are not known.

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Remains of the Wissota terrace level, here These valleys contributed sediment to, and are considered a part of the Wissota terrace, can be graded to, the main valley. traced into nearly all the valleys tributary to The lower Buffalo River valley below Mon- the lower Chippewa Valley. They slope gradu- dovi, Wisconsin, is a special case: some water ally upward in most areas, indicating that they and sediment from the lower Chippewa Valley were graded to the main valley. Lower parts was channelled into this alternate outlet during of the tributary valleys were temporarily the deposition of the Wissota terrace. Conse- ponded and filled with backwater alluvium of quently the Wissota terrace is present in the sand and silt from the main valley. This condi- preglacial lower Buffalo valley (modern Bear tion was maintained for a sufficient length of Creek valley) and also in the modern lower time to allow the flood-plain deposits of tribu- Buffalo valley. The upper Buffalo River valley tary streams to merge imperceptibly with the above Mondovi received sediment in its head- backwater filling. As a result, tributary profiles waters carried by water spilling over a low were graded to the Wissota terrace level. divide between it and the Eau Claire River Some tributary valleys, on the other hand, basin. It appears to contain a terrace level are still exceedingly flat in their lower reaches graded to the Wissota terrace, but this valley and contain sizable areas of marsh and swamp- has not been studied in detail. land, which suggest the former presence of The Wissota terrace is an outwash valley shallow lakes. No lakes are found in these val- train originating at the margin of the Chippewa leys today, however, nor are any shorelines Valley moraine of Weidman (1907, p. 493), preserved. These recent marshes are underlain which crosses the valley at Jim Falls. The by alternating layers of peat and sand, the Chippewa Valley ice lobe moved in a south- latter of local derivation. These areas have westerly direction, and movement terminated never been sufficiently filled with locally de- with the apex of the lobe located near Jim rived debris to be graded to the Wissota terrace Falls. This lobe was limited to the east by the level. Stream incision and consequent lowering minor Wisconsin Valley lobe of Weidman and of the water table have drained the shallow to the west by the St. Croix lobe. The Chip- lakes that were present earlier. pewa Valley moraine is correlated with the The formation of lakes under similar circum- Gary Stade of the Wisconsin Glaciation, a stances has been recognized by Shaw (1911; relationship established by early workers on 1915) and Thornbury (1950) in southern the basis of morphology and lithology before Illinois and southern Indiana, where glacial- the adoption of modern nomenclature. It is outwash trains in major valleys effectively recognized by Flint and others (1945) in the dammed the tributary valleys and ponded their Glacial Map of North America. lower reaches. The lake deposits of this region, Pleistocene valley fill older than the Wissota however, are composed mainly of fine silt and terrace gravel has not been recognized in the clay in contrast to the sandy deposits in the Chippewa Valley. The major Wissota accumu- Chippewa tributary valleys. lation probably took place when the ice lobe The terrace deposits of the minor tributary reached its maximum stand and stabilized its valleys are generally finer-grained than those position. The Wissota terrace seems to be of the main Chippewa Valley. They are domi- graded to the terminal moraine, and there is nantly composed of fine to coarse sand and no evidence of any extension of the terrace contain little silt and clay. In general these behind the terminal moraine. The Wissota deposits are well bedded; locally they show terrace, therefore, appears to be an outwash cross-bedding. Some of this finer debris may valley train closely related to the Gary maxi- have been swept into the mouths of tributary mum. The terrace gravel was deposited by an valleys from the major stream during alluvia- aggrading stream which spread the entire tion of the main valley, but a large amount was width of the valley at least seasonally. The probably derived locally in the tributary particle size of the deposits indicates a stream valleys themselves from the weathering prod- of considerable competence. Deficiency of fine- ucts of the friable Upper Cambrian sandstones. grained sediments suggests a lack of fine ma- The terrace deposits of the two major tribu- terial in the Gary till which furnished a source taries of the lower Chippewa Valley, the Eau for the outwash sediments. The stream velocity Claire and Red Cedar rivers, have a texture was probably sufficient to move some fine- similar to those in the main valley though they grained material through the valley. The were derived from separate glacial sources. amount of discharge of the stream was doubt-

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less greater than that of the present Chippewa with 930 feet for the Wissota terrace at this River, at least during the warmer part of the point. This terrace level is substantially below year. The braided surface pattern indicates the level of Lake Wissota (896 feet), and an that the melt water spread out over the entire earth dike has been constructed to contain the width of the valley and that it attained no lake waters at the upper end of this terrace great depth. remnant. Southwest of Chippewa Falls, a low South of Eau Claire a divide between the terrace with an elevation of about 850 feet is Eau Claire and Buffalo valleys has been observed in sees. 12 and 13, T. 28 N., R. 9 W. breached and eroded to an elevation of 1010 Farther downstream, between Chippewa Falls feet near the village of Foster. Although the and Eau Claire, a low terrace occurs along the channel through this breached divide is graded south bank of the river, sloping in elevation to the Wissota terrace surface, it is unlikely between about 740 and 730 feet in sees. 10, 11, that Wissota outwash-bearing waters actually and 15, T. 28 N., R. 9 W. Another low terrace breached the divide or even passed through the remnant with an elevation of about 850 feet Foster spillway. A water depth of at least 130 is found in sec. 27, T. 28 N., R. 9 W. In general, feet at Eau Claire would have been necessary the low terraces between Chippewa Falls and for Gary melt water to flow through this Eau Claire show only a single level at any channel. A depth of about 175 feet would have given locality, and they cannot be correlated been necessary to breach other divides in the among the different localities. area. The nature of the Wissota terrace deposits The best low-terrace development occurs for strongly suggests that no such flood of water about 10 miles along the south wall of the valley acted as an agent of their deposition; it is from the southwestern edge of the city of Eau likely that the Foster spillway and the breached Claire to Caryville, Dunn County. Two or divide in the Buffalo Valley south of Mondovi more low-terrace levels are exposed at several were the result of erosion by melt water ponded places, and they, together with the higher in front of the Farmdale ice sheet, which had Wissota terrace, give a "stair-step" effect to the penetrated into the region earlier. topography. Figure 4 is a sketch map showing The deposition of the Wissota terrace was the geographic extent of the low-terrace rem- terminated by: (1) the retreat of the Chippewa nants and their approximate elevations. En- Valley ice lobe which removed the abundant trenchment of the tributary streams is ignored source of fresh sediment from the system and on this map, and the Wissota terrace is not (2) the retreat of the Superior and Des Moines differentiated from the bedrock. A part of this lobes which removed the supply of sediment area is shown in Figure 2 of Plate 1, an aerial from the Mississippi drainage system. Con- photograph of the multiple terrace levels east tinued abundant melt water was supplied to of Caryville. These low terraces range in eleva- the latter system, and entrenchment of the tion from just below the Wissota terrace to a Gary outwash deposits in the Mississippi Valley few feet above the present flood plain. The followed. The lowering of the outlet of the various levels seem to be discontinuous, and no Chippewa Valley, as well as an abundant flow secondary low-terrace system has been identi- of melt water in the Chippewa Valley itself, fied. Southwest of Caryville a low terrace is caused entrenchment which converted the recognized in sees. 13 and 14, T. 26 N., R. 12 Gary outwash valley train into the Wissota W. near Meridean. This terrace has an average terrace. elevation of about 750 feet and slopes gently into the present flood plain. At least one low- Low Terraces terrace level occurs in the tributary Red Cedar The low terraces of the Chippewa Valley are valley, but it has been not studied. lower in elevation than the Wissota terrace but The low terraces generally occur as smoothly higher than the present flood plain. The rounded notches cut into a higher terrace level. distribution of the recognized low-terrace levels The escarpment of the higher terrace (either in the valley is shown in Figure 3. None have the Wissota terrace or a higher low terrace) is been noted between the Gary terminal moraine commonly steep and probably approaches the at Jim Falls and the outlet of Lake Wissota. angle of repose of the gravel. Sheet wash and east of Chippewa Falls. Immediately north of incipient streams are not very effective in ero- the Northern States Power Company dam, sion of this permeable deposit. Smoothly round- which forms Lake Wissota, there is a low terrace ed escarpments and occasional cusps suggest at an elevation of about 880 feet, compared that the low terraces were cut by meandering

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Figure 1. Vertical aerial photograph showing the oriented, braided pattern on the surface of the Wissota terrace tread, T. 27 N., R. 11 W., Dunn County, Wisconsin. U.S. Dept. of Agriculture photo BRT-9V-39

Figure 2. Vertical aerial photograph of the Wissota and low terraces east of Caryville, Wisconsin, T. 26 N., R. 11 W., Dunn County, and in T. 26 N., R. 10 W., Eau Claire County. U.S. Dept, of Agriculture photo BRT-9V-127

TERRACES OF THE LOWER CHIPPEWA VALLEY, WISCONSIN

ANDREWS, PLATE 1 Geological Society of America Bulletin, volume 76

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streams. A crescentic pattern of minor ridges they may have been buried by more recent and swales analogous to the point-bar deposits deposition of debris eroded from the upper part of the modern meandering Chippewa River is of the valley. The low terraces of the valley noted on the surface of some of the low terraces cannot positively be correlated at definite levels paralleling their rounded outline. Some of the and are typical unpaired terraces, developed low-terrace remnants seem to slope gently during the excavation of the valley by a mean- toward the valley wall, indicating the last dering stream. stand of the former stream channel in this position. Early Wisconsin Glaciation The composition of the low terraces, as far Deposits of pre-Cary drift in St. Croix, as can be observed, is virtually identical with Pierce and Dunn counties, Wisconsin, were

».ll W.IR.1QW Figure 4. Distribution and elevation of low-terrace remnants between Eau Claire and Caryville, Wisconsin

that of the Wissota terrace, suggesting that first described by Chamberlin (1910), and they the low terraces have an erosional origin and have been more recently correlated by Black that the deposits are mainly Wissota terrace (1959, p. 8-9) as Farmdale in age. Black has sediments. The upper few feet of the lower also reported the occurrence of drift of Farm- terraces should show some modification from dale age south and west of Eau Claire on both the typical Wissota sediments because the re- sides of the Chippewa River. Gravel mounds worked deposits of the low terraces were largely and thin drift, probably deposited by an Early derived from Wissota terrace sediments, but Wisconsin ice sheet, have also been observed the differences might not be great enough to be in T. 26 N., R. 11 W., northwest of Rock Falls. diagnostic. The influence on the Chippewa Valley of the The low terraces of the Chippewa Valley ap- ice sheet which deposited this drift is unknown. pear to be the results of erosion by meandering Some Pleistocene drainage changes in the streams which dissected the older Wissota ter- lower Chippewa Valley region are indicated in race. Some local redeposition occurred in the Figure 5. The diversion of the lower part of the formation of point bars, but the net result was Buffalo River valley has been discussed by removal of sediment from the valley or re- Martin (1932, p. 199-201). The occurrence of deposition in its lower reaches. Low terraces igneous boulders and other glacial debris in the have not been observed in the farthest down- ancient lower valley of the Buffalo River in the stream segment of the Chippewa Valley, but vicinity of sec. 36, T. 25 N., R. 12 W. suggests

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that an early ice sheet, possibly of Farmdale and Buffalo rivers. The Red Cedar River age, may have blocked the valley, causing the drained most of the interlobate area between Buffalo River to be diverted through a low the St. Croix and Chippewa Valley ice lobes, divide on the south valley wall. The diversion and the other two rivers received melt water of the lower Red Cedar River (see Fig. 5) is and glacial debris from their headwaters. The more probably the result of the stream follow- entire lower Chippewa drainage system appears ing a steeper gradient on the Wissota terrace to have been filled with sediment to the Wissota tread than the disruption of pre-glacial drainage terrace level, except for ponded areas in some by an Early Wisconsin ice sheet. minor tributary valleys. The Mississippi Valley

Figure 5. Preglacial drainage of the lower Chippewa Valley region and Pleistocene stream diversions, west-central Wisconsin. Arrows indicate Pleistocene drainage changes

near the mouth of the Chippewa River was Gary Stade History probably also filled to a corresponding level. The ice advance of the Gary Stade was the most extensive late Wisconsin advance in the Man\ato State ofLeighton, 1931, History upper Mississippi Valley region. In the Chip- Erosion of the Wissota terrace undoubtedly pewa Valley the valley-outwash train of the took place in post-Gary, pre-Mankato time, Wissota terrace level was deposited. The Wisso- but no evidence of this event is observed in the ta terrace gravel was probably deposited when Chippewa Valley. Probably only a little or no the glacier approached and stabilized its posi- Mankato melt water reached the Chippewa tion at the terminal moraine. Sediment was Valley. brought into the valley mainly from the area Debris-laden melt water from the Des where the Chippewa River crosses the Gary Moines lobe and the Grantsburg sublobe de- terminal moraine, but some sediment was sup- posited a Mankato terrace in the Mississippi plied from the headwaters of three major Valley which has been mapped by Ruhe and tributary streams: the Red Cedar, Eau Claire, Gould (1954) in Dakota County, Minnesota.

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The Mankato terrace level of this area can be able to supply it. Debris from the Chippewa projected to remnants down the Mississippi River began to accumulate as a delta in the Valley. Near the mouth of the Chippewa lower reaches of the Chippewa Valley and River this terrace seems to be at an elevation downstream in the Mississippi Valley. This delta of about 730 feet. This coincides with the 730- dammed the Mississippi Valley to form Lake foot terrace level of the left bank in the lower- Pepin, as discussed by Martin (1932, p. 169- most Chippewa Valley and suggests that this 171) and Zumberge (1952, p. 32-40). The up- level was controlled by Mankato deposition in stream extent and depth of entrenchment of the main valley. No correlative of the Mankato the Chippewa River into the Wissota terrace terrace level has been identified farther up the deposits at the inception of the Recent regimen Chippewa Valley. is unknown. Between Caryville and Chippewa Falls low terraces were cut probably from post- Valders Stade History Gary time until the present. Vertical erosion A low outlet for the Chippewa Valley was progressed more rapidly at first when the probably maintained from the time of retreat Chippewa Valley had a higher gradient. Be- of the Mankato ice and throughout the Valders cause of lack of data, glacial and postglacial Stade. It is not thought that any Valders melt crustal warping of the region has been ignored. water spilled into the Chippewa drainage sys- Figure 3 shows a profile of the modern tem, but abundant debris-free melt water with Chippewa Valley based on elevations projected considerable erosive power was channelled to straight-line longitudinal sections of the through the Mississippi Valley during Valders valley; stream length is not shown. The lower and post-Valders time. Erosion of the bed of the Chippewa Valley may be divided into three Mississippi River probably resulted, with a segments: the lowest extending from the consequent lowering of the Chippewa River mouth of the river to the base of the first outlet. The elevation of this low outlet should meander, Cranes Bend, about 1 mile north of approximate the maximum depth of Lake Durand; the central one extending from Cranes Pepin, the basin of which received its final Bend to the Sterling Paper and Pulp Company shaping at this time. Martin (1932, p. 169) gives dam at Eau Claire; and the third extending the maximum depth of Lake Pepin as 56 feet, from Eau Claire to the dam at Jim Falls, where which occurs near the lower end of the lake and the river crosses the Gary terminal moraine. hence near the mouth of the Chippewa River. The lowest segment of the river has a com- Barring an appreciable accumulation of more paratively straight course, is rather broad and recent sediment here, this suggests a post- shallow, and contains numerous small islands Valders outlet elevation of about 610 feet for and sand bars. The length of this segment of the Chippewa River, in contrast to the present the river is 18.6 miles, and the gradient aver- outlet elevation of 667 feet. ages 1.77 feet per mile. This segment of the valley is about 18 miles long, and the average Postglacial History gradient of the valley floor is about 1.83 feet The present regimen of the Chippewa River per mile (see Fig. 4). This segment of the was initiated in the post-Algonquin stage of Chippewa Valley contains an aggrading, braid- development of the Great Lakes (see Hough, ed stream unable to transport all the debris 1958, Fig. 71). The opening of an eastern outlet furnished from upstream. This segment has for the Lake Duluth basin finally ended drain- been extended headward as the delta has accu- age of glacial melt water from Lake Duluth and mulated in the lower part of the valley. Mean- from Lake Agassiz 2. Thus, the discharge into der scars on the valley walls south of Durand the Mississippi Valley was substantially de- indicate the former downstream extension of creased and was probably not much greater the meandering segment into this area. The than at present. The Mississippi Valley, how- lowest segment of the Chippewa River flows ever, had adjusted its gradient to a much great- exclusively on its own Recent sediments ex- er flow of water, whereas the Chippewa Valley cept where it impinges on Cambrian sandstone had not been so adjusted. Consequently, the and Wissota terrace sediments along the valley Chippewa River continued to erode its bed walls. and deposit sediment at its mouth. The Missis- The central segment of the river has a sinuous sippi River, with a reduced volume and low and meandering course, and its sand bars are gradient, was no longer able to remove this related to point-bar development. The length sediment as rapidly as the Chippewa River was of this segment of the river is 44 miles, and the

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gradient averages 1.77 feet per mile. The Valley with the Mississippi Valley. The lower length of this segment of the valley is 27.6 miles, Chippewa Valley was filled as much as 200 feet and the average gradient of the valley floor is with a valley train of Gary outwash gravel. 2.46 feet per mile. The central segment of the This deposit seems to be graded with regard to lower Chippewa River is a typical meandering the stream regimen at the time of deposition, stream with vertical erosion restricted by the and most of the tributary valleys are also graded slowly rising base level formed by the accumu- to this level. The Gary outwash deposits have lating debris in the lower segment of the valley. been partly removed by subsequent erosion, The gradient of the central segment of the resulting in the formation of the Wissota valley floor is substantially greater than that of terrace. This terrace is a prominent geomorphic the lower segment, but the stream has adjusted feature, and the terrace level can be traced its velocity by an increase in length so that the with certainty throughout the main valley and stream gradient of the central meandering the tributary valleys. A sequence of erosional segment is practically the same as that of the low terraces are cut in Wissota terrace deposits lower braiding segment. The central segment in part of the valley. They have not yet, how- of the river flows on Recent and Wissota ter- ever, been correlated with definite events in the race alluvium, except for impinging oc- history of the valley. casionally on the bedrock exposed in the valley Deposition of the Gary outwash sediments walls. was terminated by the retreat of the ice sheet, The third segment of the river is more diffi- and abundant melt water eroded the Mississippi cult to classify; parts of it are submerged by the Valley, with a consequent lowering of the outlet waters of three reservoirs. It flows mainly of the Chippewa Valley. Deposition of a Man- through a bedrock channel which is partly kato outwash train in the Mississippi Valley composed of resistant granite that has not been raised the outlet of the Chippewa Valley to an appreciably eroded in post-Pleistocene time. elevation of about 730 feet. Late glacial melt The stream profile is therefore not smooth and water probably maintained a low outlet for the is obscured by the damming of the stream. The Chippewa Valley by eroding the Mississippi inferred profile of the original stream channel is Valley. The final post-Valders outlet elevation indicated by the dashed lines on Figure 3. The is thought to have been about 610 feet or lower. length of this segment of the river is 30.7 miles, The draining of glacial Lake Duluth into the and the gradient averages 4.77 feet per mile. Lake Michigan basin initiated the Recent The length of this segment of the valley is regimen for the valley. The modern Chippewa about 19.1 miles, and the average gradient of River has built a delta at its mouth and is the valley floor is about 6.70 feet per mile. aggrading the lower 18 miles of the valley; Aluch of this drop in elevation occurs at Chip- the outlet elevation at present is 667 feet. The pewa Falls, so that these average figures are not central part of the valley contains a meandering particularly meaningful. The course of this stream which is actively eroding horizontally. upper segment is restricted by partial entrench- The upper segment of the stream is partly ment in bedrock, but where the stream has entrenched in granitic bedrock, and its level is crossed areas of unconsolidated sediments, maintained by bedrock sills. An understanding lateral as well as vertical erosion has occurred. of these three segments of the present stream is Old meander scars cut in the Wissota terrace helpful in the interpretation of the post-Gary gravels are found between F.au Claire and erosional and depositional history of the lower Chippewa Falls and at Lake Wissota. This seg- Chippewa Valley. ment of the stream prior to damming was reducing its channel primarily by vertical Acknowledgments downcutting, but this downcutting was im- The writer expresses his appreciation to peded by hard bedrock sills. George F. Hanson, State Geologist of Wis- consin, for his co-operation in making available Summary unpublished data in the files of the Wisconsin The lower Chippewa Valley is a distinct Geological Survey. Helpful suggestions from geomorphic region which extends from the Sheldon Judson, Princeton University, and terminal moraine of the Chippewa Valley lobe Louis L. Ray and Lynn A. Yehle, U. S. Geo- of the Gary ice sheet in Chippewa County, logical Survey, are much appreciated. Wisconsin, to the confluence of the Chippewa

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References Cited Black, Robert F., 1959, Glacial geology of west-central Wisconsin: Guidebook, 10th Ann. Field Conf., Midwestern Friends of the Pleistocene, 14 p. Chamberlin, Rollin T., 1910, Older drifts in the St. Croix region: Jour. Geology, v. 18, p. 542-548 Chamberlin, T. C., and Salisbury, R. D., 1885, Preliminary paper on the driftless area of the upper Mississippi Valley: U.S. Geol. Survey, 6th Ann. Rept., p. 199-322 Flint, Richard Foster, and others, 1945, Glacial map of North America: Geol. Soc. America Special Paper 60 Hough, Jack L., 1958, Geology of the Great Lakes: Urbana, Univ. Illinois Press, 313 p. Leighton, Morris M., 1931, The Peorian loess and the classification of the glacial drift sheets of the Mississippi Valley: Jour. Geology, v. 39, p. 45-53 Martin, Lawrence, 1932, The physical geography of Wisconsin, 2d edition: Wis. Geol. Survey Bull. 36, 608 p. Owen, David Dale, 1848, Report of a geological reconnoissance [sic] of the Chippewa land district of Wisconsin: U.S. 30th Cong., 1st Sess., Senate Ex. Doc. 57, p. 1-72 Ruhe, Robert V., and Gould, Laurence M., 1954, Glacial geology of the Dakota County area, Minnesota: Geol. Soc. America Bull., v. 65, p. 769-792 Shaw, E. W., 1911, Preliminary statement concerning a new system of Quaternary lakes in the Mississippi basin: Jour. Geology, v. 19, p. 481-491 1915, Newly discovered beds of extinct lakes in southern and western Illinois and adjacent states: 111. State Geol. Survey Bull. 20, p. 139-157 Thornbury, William D., 1950, Glacial sluiceways and lacustrine plains of southern Indiana: Ind. Dept. Conserv., Div. Geology Bull. 4, 21 p. Weidman, Samuel, 1907, The geology of north-central Wisconsin: Wis. Geol. Survey Bull. 16, 697 p. Wooster, L. C., 1882, Geology of the lower St. Croix district: Geology of W'is., v. 4, p. 99-159 Zumberge, James H., 1952, The lakes of Minnesota, their origin and classification: Minn. Geol. Survey Bull. 35, 99 p.

U. S. GEOLOGICAL SURVEY, WASHINGTON, D. C. MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 6, 1963 PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY

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