James C. Knox

Sensitivity of modern and Holocene floods to climate change

Abstract Alluvial records of paleofloods show that natural floods resulting from excessive rainfall, snowmelt, or from combined rainfall and snowmelt are highly sensitive to even modest changes of climate equivalent or smaller than changes expected from potential future global warming in the 21st century. The high sensitivity results from effects of hemispheric or global-scale changes in circulation patterns of the ocean and atmosphere to influence the pathways and locations of air masses and storm tracks. Holocene paleoflood chronologies from the Upper Mississippi Valley in the Midwest United States and from the Colorado River drainage of the Southwest United States show that recurrence frequencies of large floods have been subject to abrupt changes over time. These flood chronologies and flood chronologies observed for other middle-latitude regions suggest that recurrence frequencies of large floods are increased when there is an increase in the number of waves and their amplitudes in the middle and upper tropospheric circum-polar westerly circulation. However, some middle-latitude regions on the western margins of continents experience increased frequencies of flooding during strong onshore zonal westerly circulation. Flood chronologies from several regions suggest that times of rapid climate change have a tendency to be associated with more frequent occurrences of large and extreme floods. The unusual high frequencies of large floods that have been observed in many regions since the early 1950s are often attributed to land use change, but the rapid climate forcing from the effects of increased atmospheric greenhouse gases may also be a contributing factor. Paleoflood records provide information that is useful for better interpretation and calibration of modern short-term instrumental records, and they provide unique event-scale information that is useful for calibrating and testing geophysical models of past and anticipated future climate conditions.

Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley

Abstract Agricultural landscapes are more sensitive to climatic variability than natural landscapes because tillage and grazing typically reduce water infiltration and increase rates and magnitudes of surface runoff. This paper evaluates how agricultural land use influenced the relative responsiveness of floods, erosion, and sedimentation to extreme and nonextreme hydrologic activity occurring in watersheds of the Upper Mississippi Valley. Temporally overlapping stratigraphic and historical instrumental records from southwestern Wisconsin and northwestern Illinois show how agricultural modification of a natural prairie and forest land cover affected the behavior of floods and sedimentation during the last two centuries. For comparison, pre-agriculture Holocene alluvial sediments document the sensitivity of floods and alluvial activity to climate change prior to significant human influences on the natural land cover. High-resolution floodplain stratigraphy of the last two centuries shows that accelerated runoff associated with agricultural land use has increased the magnitudes of floods across a wide range of recurrence frequencies. The stratigraphic record also shows that large floods have been particularly important to the movement and storage of sediment in the floodplains of the Upper Mississippi Valley. Comparison of floodplain alluvial sequences in watersheds ranging in scale from headwater tributaries to the main valley Mississippi River demonstrates that land use changes triggered hydrologic responses that were transmitted nearly simultaneously to all watershed scales. In turn, flood-driven hydraulic adjustments in channel and floodplain morphologies contributed to feedback effects that caused scale-dependent long-term lag responses. There has been a general reduction in magnitudes of flooding, erosion, and sedimentation since the mid-20th century, largely in response to better land conservation practices. The reduction trend is most apparent on tributary watersheds of a few hundred square kilometers and smaller sizes. However, the main-channel Upper Mississippi River, with associated drainage areas between about 100,000–200,000 km2, has experienced increased occurrences of large floods during the second half of the 20th century. Most of these large floods have been associated with snowmelt runoff which is occurring more rapidly and earlier in the season in response to a trend toward warmer winters and springs in the late 20th century. Modification of the natural drainage network through establishment of drainage tiles and channelization has also continued during the late 20th century. Tiling and channelization have increased drainage efficiency and probably have contributed in part to the occurrence of large floods on the Mississippi River, but the magnitudes of their effects are unknown at present. In spite of reduced sediment loads since about 1950 on all watershed scales, the anomalous high frequency of large floods on the Upper Mississippi River continues the accelerated delivery of agriculturally-related sediment to floodplain and backwater environments. The results of this study indicate that agricultural land use has escalated landscape sensitivity to such a degree that modern process rates provide a very distorted representation of process rates that occurred in the geologic past prior to human disturbance.

Responses of floods to Holocene climatic change in the upper Mississippi Valley

Abstract Dimensions of Holocene relict channels and sedimentological characteristics of point bars associated with these relict channels were used to reconstruct a Holocene history of long-term changes in magnitudes of 1.58-yr floods in Upper Mississippi Valley watersheds of southwestern Wisconsin. The reconstructed record of floods shows relatively large and persistent (nonrandom) departures from contemporary long-term average flood magnitudes. The flood history indicates climatic changes that are broadly similar to climatic changes indicated from fossil pollen in the same region. The Holocene floods ranged from about 10–15% larger to 20–30% smaller than contemporary floods of the same recurrence frequency. Large floods were characteristic between about 6000 – 4500 and 3000 – 2000 yr B.P., and during a brief interval after 1200 yr B.P. Small floods were common between about 8000 – 6500, 4500 – 3000, and 2000 – 1200 yr B.P. These fluvial responses were found to be closely associated with a long-term episodic mobility and storage of sediments in the Wisconsin watersheds. During periods of relatively large floods, relatively rapid lateral channel migration either reworked or removed extensive tracts of valley bottom alluvium. In contrast, during periods of relatively small floods, relatively slow lateral channel migration is apparent and the channel and floodplain system appear to have been relatively stable.

Paleosols and the forest border in Keewatin, N.W.T.

Abstract The morphology of paleosols and radiocarbon-dated charcoal from buried surface horizons of soils provide evidence to suggest that between periods of northward forest encroachment tundra climate has dominated areas at least 50 km south of the present forest/tundra border in southwest Keewatin. The present forest/tundra border climate is nearly as severe as any climate that has prevailed in the area since deglaciation.

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

Abstract The period in the Upper Mississippi Valley (UMV) from about 25 000 years B.P. until the time of strong human influence on the landscape beginning about 150–200 years ago can be characterized by three distinctly different alluvial episodes. The first episode is dominated by the direct and indirect effects of Late Wisconsin glacial ice in the basin headwaters. This period, which lasted until about 14 000 years B.P., was generally a time of progressive valley aggradation by a braided river system transporting large quantities of bedload sediment. An island braided system evolved during the second episode, which extended from about 14 000 to 9000 years B.P. The second episode is associated with major environmental changes of deglaciation when occurrences of major floods and sustained flows of low sediment concentration from drainage of proglacial lakes produced major downcutting. By the time of the beginning of the third episode about 9000 years B.P., most vegetation communities had established their approximate average Holocene locations. The change of climate and establishment of good vegetation cover caused upland landscapes of the UMV to become relatively stable during the Holocene in comparison to their relative instability during the Late Wisconsin. However, Holocene remobilization of Late Wisconsin age sediment stored in tributary valleys resulted in a return to long-term upper Mississippi River aggradation. The dominance of Holocene deposition over transportation reflects the abundance of sandy bedload sediment introduced from tributaries and the situation that energy conditions for floods and the hydraulic gradient of the upper Mississippi River are much less for the Holocene than they were for the Late Wisconsin and deglaciation periods. Outburst floods from glacial lakes appear to have been common in the UMV during the Late Wisconsin and especially during deglaciation. Magnitudes for the Late Wisconsin floods are generally poorly understood, but an estimate of 10 000–15 000 m3 s−1 was determined for one of the largest events in the northern UMV based on heights of paleo-foreset beds in a flood unit deposited in the Savanna Terrace. For comparison, the great flood of 1993 on the upper Mississippi River was about 12 000 m3 s−1 at Keokuk, Iowa, near the Des Moines River confluence where it represented the 500-year event in relation to modem flood series. Exceptionally large outburst floods derived from the rapid drainage of pro-glacial Lake Michigan and adjacent smaller proglacial lakes between about 16 000 and 15 500 years B.P. are a likely cause of the final diversion of the Mississippi River through the Bell City-Oran Gap at the upstream end of the Lower Mississippi Valley (LMV). The largest outburst flood from northern extremities of the UMV appears to have occurred between about 11700 and 10 800 years B.P. when the southern outlet of Lake Agassiz was incised. Based on the probable maximum capacity of the Agassiz flood channel 600 km downstream near the junction of the Wisconsin and Mississippi Rivers, the Agassiz flood discharge apparently did not exceed 30 000 m3 s−1. However, if the Agassiz flood channel here is expanded to include an incised component, then the flood discharge maximum could have been as large as 100,000 to 125 000 m3 s−1. The larger flood is presently viewed as unlikely, however, because field evidence suggests that the incised component of the cross-section probably developed after the main Agassiz flood event. Nevertheless, the large Agassiz flood between about 11 700 and 10 800 years B.P. produced major erosional downcutting and removal of Late Wisconsin sediment in the UMV. This flood also appears to be mainly responsible for the final diversion of the Mississippi River through Thebes Gap in extreme southwestern Illinois and the formation of the Charleston alluvial fan at the head of the LMV. After about 9000 years B.P. prairie-forest ecotones with associated steep seasonal climatic boundaries were established across the northern and southern regions of the UMV. The general presence of these steep climatically sensitive boundaries throughout the Holocene, in concert with the natural tendency for grasslands to be especially sensitive to climatic change, may partially explain why widespread synchroneity of Holocene alluvial episodes is recognized across the upper Mississippi River and Missouri River drainage systems. Comparison of estimated beginning ages of Holocene flood episodes and alluvial chronologies for upper Mississippi River and Missouri River systems with beginning ages for LMV meander belts and delta lobes shows a relatively strong correlation. At present, dating controls are not sufficiently adequate and confidence intervals associated with the identified ages representing system changes are too large to establish firm causal connections. Although the limitations of the existing data are numerous, the implicit causal connections suggested from existing information suggest that further exploration would be beneficial to improving the understanding of how upper valley hydrological and geomorphic events are influencing hydrological and geomorphic activity in the LMV. Since nearly 80% of the Mississippi River drainage system lies upstream of the confluence of the Mississippi and Ohio Rivers, there is a strong basis for supporting the idea that UMV fluvial activity should be having a strong influence on LMV fluvial activity. If this assertion is correct, then the traditional assignment of strong to dominant control by eustatic sea level variations for explaining channel avulsions, delta lobes, and meander belts in the LMV needs re-examination. A stronger role for upper valley fluvial activity as a factor influencing lower valley fluvial activity does not disregard the role of eustatic sea level, tectonic processes or other factors. Rather, upper valley fluvial episodes or specific events such as extreme floods may commonly serve as a “triggering mechanism” that causes a threshold of instability to be exceeded in a system that was poised for change due to sea level rise, tectonic uplift, or other environmental factors. In other situations, the upper valley fluvial activity may exert a more dominant control over many LMV fluvial processes and landforms as frequently was the case during times of glacial climatic conditions.

SPATIAL AND TEMPORAL SENSITIVITY OF HYDROGEOMORPHIC RESPONSE AND RECOVERY TO DEFORESTATION, AGRICULTURE, AND FLOODS

Clear-cut logging followed by agricultural activity caused hydrologic and geomorphic changes in North Fish Creek, a Wisconsin tributary to Lake Superior. Hydrogeomorphic responses to changes in land use were sensitive to the location of reaches along the main stem and to the relative timing of large floods. Hydrologic and sediment-load modeling indicates that flood peaks were three times larger and sediment loads were five times larger during maximum agricultural activity in the 1920s and 1930s than prior to about 1890, when forest cover was dominant. Following logging, overbank sedimentation rates in the lower main stem increased four to six times above pre-settlement rates. Accelerated streambank and channel erosion in the upper main stem have been and continue to be primary sources of sediment to downstream reaches. Extreme floods in 1941 and 1946, followed by frequent moderate floods through 1954, caused extensive geomorphic changes along the entire main stem. Sedimentation rates in the lower main stem may have decreased in the last several decades as agricultural activity declined. However, geomorphic recovery is slow, as incised channels in the upper main stem function as efficient conveyors of watershed surface runoff and thereby continue to promote flooding and sedimentation problems downstream. [Key words: fluvial geomorphology, floods, erosion, sedimentation, deforestation, agriculture.]

Age of colluvium indicates accelerated late Wisconsinan hillslope erosion in the Upper Mississippi Valley

Colluvium on foot slopes in parts of North America and Europe has been attributed to a major mass-wasting episode during the last glacial period. Stratigraphic evidence and 14 C ages support this hypothesis for the northern part of the Upper Mississippi Valley. Colluvium in this region grades laterally into, or interfingers with, fluvial sediment beneath the late Wisconsinan Savanna terrace. Colluvial foot slopes are truncated by fluvial surfaces postdating the incision that created the Savanna terrace between 13 and 11 ka. Samples from within the colluvium have 14 C ages between 18.6 and 12 ka. Ages of 28.9 and 20.3 ka have been obtained from beneath colluvium, and fluvial sediment inset into colluvial foot slopes has yielded ages between 12.5 and 9.8 ka. Widespread permafrost during the late Wisconsinan glacial maximum may explain the onset of accelerated mass wasting that produced the colluvium, although mass wasting apparently continued for some time during subsequent climatic warming. The results described here imply major, long-term, climatically driven fluctuations in sediment supply from hillslopes to the fluvial system in this region.

Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah

Long-term variations in Holocene fl ood magnitude were quantifi ed from the bankfull dimensions of abandoned channels preserved on flsurfaces in the northern Uinta Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were reconstructed, and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross-section areas. The results indicate systematic (nonrandom) variations of bankfull fl oods in the northern Uinta Mountains. Large fl oods, as much as 10%‐15% greater than modern, dominated from 8500 to 5000 calendar yr B.P., and again from 2800 to 1000 cal yr B.P. Small fl oods, as much as 15%‐20% less than modern, characterize the periods from 5000 to 2800 cal yr B.P., and from 1000 cal yr B.P. to near present. The middle and late Holocene record of bankfull fl ood magnitude compares well with independent evidence for climatic variation in the area. The early Holocene record indicates that larger than modern bankfull fl oods coincide with warmer than modern mean annual temperature. We hypothesize that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid melting of deep snowpacks in the high Uinta Mountains, thus producing large fl oods despite warmer mean annual temperatures. The episode of smaller than modern bankfull fl oods between 5000 and 2800 cal yr B.P. coincides with records of increased forest fi re frequency in the northern Uintas. Larger than modern fl oods from 2800 to 1000 cal yr B.P. coincide with a local decrease in forest fi re frequency and evidence for minor local glacial readvances. The decrease in fl magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly.

Provenance and pedology of a long-term Pleistocene depositional sequence in Wisconsin's Driftless Area

Abstract Soil coring in the Driftless Area of Wisconsin has revealed an approximately 9 m stratigraphic sequence containing five buried soils formed in seven lithologic units below the youngest late Wisconsin loess. The sequence is preserved on a toeslope remnant, 30 m above the present valley floor, and provides the most complete stratigraphic section known for the Upper Mississippi Valley region. The stratigraphic sequence is

Micromorphology of a “welded” Sangamonian to Wisconsinan age paleosol in southwestern Wisconsin

Abstract Micromorphological characteristics indicate that in southwestern Wisconsin pedogenesis transgressed the Sangamonian-Wisconsinan chronostratigraphic boundary in conjunction with colluvial and eolian sedimentation. The use of micromorphology helps to distinguish between pedological, colluvial, and eolian components in the paleosol horizon sequence, which can be difficult to resolve and interpret solely from field investigations and from other laboratory analyses. Results show that a basal loess “mixed zone” within the welded paleosol profile contains features that are the products of colluvial reworking processes. We present micromorphology data which support stratigraphic relationships suggesting that colluvial processes were important in the formation of basal loess “mixed zones” in addition to other possible mixing processes such as pedoturbation and bioturbation.