No abstract

Our study adds to the Quaternary history of eolian systems and deposits in western Wisconsin, USA, primarily within the lower Chippewa River valley. Thickness and textural patterns of loess deposits in the region indicate transport by west-northwesterly and westerly winds. Loess is thickest and coarsest on the southeastern flanks of large bedrock ridges and uplands, similar in some ways to shadow dunes. In many areas, sand was transported up and onto the western flanks of bedrock ridges as sand ramps, presumably as loess was deposited in their lee. Long, linear dunes, common on the sandy lowlands of the Chippewa valley, also trend to the east-southeast. Small depressional blowouts are widespread here as well and often lie immediately upwind of small parabolic dunes. Finally, in areas where sediment was being exposed by erosion along cutbanks of the Chippewa River, sand appears to have been transported up and onto the terrace treads, forming cliff-top dunes. Luminescence data indicate that this activity has continued throughout the latest Pleistocene and into the mid-Holocene. Together, these landforms and sediments paint a picture of a locally destabilized landscape with widespread eolian activity throughout much of the postglacial period.

This study employs data from valley-bottom surveys, coring investigations, topographic maps, and aerial photographs to identify and explain spatial variations in historical alluviation along tributary streams in the Buffalo River watershed, an agricultural watershed in west-central Wisconsin. Similar to findings from other agricultural watersheds, the spatial distribution of historical alluvium in the Buffalo watershed reflects the controlling influence of watershed size and valley-bottom width. These two factors are not, however, clearly related to the deposition and accumulation of historical sediment at all sites. An additional significant factor that has affected valley-bottom sedimentation is historical channel incision. Incision of nearly every tributary stream in the watershed has created enlarged channels with considerable capacities to contain floods, keeping them from inundating and depositing sediment on valley floors. This helps explain why relatively large amounts of historical alluvium were deposited along some streams (because they are unincised or were incised relatively recently) and why much less sediment was deposited along others despite similar watershed characteristics (because they were incised early in the historical period). It also helps explain why, along some reaches, quantities of historical alluvium do not decrease in the upstream direction as expected. Apparently, incision by upstream-advancing headcuts allowed more time for alluvium to accumulate at upstream sites, countering the effects of decreasing drainage area. Today, tributary streams in the Buffalo River watershed compose a fairly well-integrated network of incised channels. As a result, tributary streams can now convey floods and sediment downstream more efficiently than they could in the past.

The Buffalo River is a tributary to the Mississippi River in west‐central Wisconsin that drains a watershed dominated by agricultural land uses. Since 1935, backwater from Lock and Dam 4 on the Mississippi River has inundated the mouth of the Buffalo's valley. Resurveys of a transect first surveyed across the lake in 1935 and cesium‐137 dating of backwater sediments reveal that sedimentation rates at the Buffalo's mouth have remained unchanged since the mid‐1940s. Study results indicate that sediment yields from the watershed have persisted at relatively high levels over a period of several decades despite pronounced trends toward less cultivated land and major efforts to control soil erosion from agricultural land. The maintenance of sediment yields is probably due to increased channel conveyance capacities resulting from incision along some tributary streams since the early 1950s. Post‐1950 incision extended the network of historical incised tributary channels, enhancing the efficient delivery of sediment from upland sources to downstream sites.

Large parts of the upper Midwest, USA were impacted by permafrost during the Last Glacial Maximum (LGM). Even though permafrost persisted as the Laurentide Ice Sheet began to recede, direct age control of this interval is largely lacking. To better temporally constrain the permafrost interval in western Wisconsin, we identified two sites, outside the Late Wisconsin (MIS 2) glacial limit, that contain relict, ice‐wedge pseudomorphs, initially interpreted to be sand wedges, hosted within well‐drained outwash deposits. The pre‐Wisconsin (>MIS 5) host material commonly displays up‐turned bedding near the contact with the wedges, indicative of well‐formed features. The wedges are filled with well‐sorted, gravel‐free, medium and fine sands, and lack evidence of post‐formational disturbance, pointing to an aeolian sand infill and confirming them as sand wedges. Ventifacts on nearby uplands attest to windy conditions here in the past. Optically stimulated luminescence (OSL) ages on five sand wedges indicate that they filled with sand between c. 19.3 and 18.3 ka at the southerly site and between c. 15.1 and 14.7 ka at the northerly site, which is closer to the LGM margin. Sand wedges at the latter site were wider and had more complex morphologies, possibly suggesting a longer interval of formation and/or more intense permafrost. We also examined a site along a ridge crest, between the two wedge sites, which displayed interbedded loess and sand, dated by OSL to 12.7 ka. Together, these results point to dry, cold, windy conditions in west‐central Wisconsin, within 100 km of the LGM limit. At this time, aeolian sands were being transported across a landscape with (at least scattered) permafrost. The OSL results suggest multiple phases, or perhaps time‐transgressive, sand‐wedge formation, associated with permafrost between c. 19 and 15 ka, with dry, windy (and likely, cold) conditions persisting until at least 12.7 ka.