Conversion of upland forest and prairie vegetation to agricultural land uses, following Euro-American settlement in the Upper Mississippi River System, led to accelerated runoff and soil erosion that subsequently transformed channels, floodplains, and wetlands on bottomlands. Halfway Creek Marsh, at the junction of Halfway Creek and the Mississippi River on Wisconsin’s western border, is representative of such historical transformation. This marsh became the focus of a 2005–2006 investigation by scientists from the U.S. Geological Survey, the University of Wisconsin–Madison, and the U.S. Environmental Protection Agency, who used an understanding of the historical transformation to help managers identify possible restoration alternatives for Halfway Creek Marsh. Field-scale topographic surveys and sediment cores provided data for reconstructing patterns and rates of historical overbank sedimentation in the marsh. Information culled from historical maps, aerial photographs, General Land Office Survey notes, and other historical documents helped establish the timing of anthropogenic disturbances and document changes in channel patterns. Major human disturbances, in addition to agricultural land uses, included railroad and road building, construction of artificial levees, drainage alterations, and repeated dam failures associated with large floods. A volume of approximately 1,400,000 m 3 , involving up to 2 m of sandy historical overbank deposition, is stored through the upper and lower marshes and along the adjacent margins of Halfway Creek and its principal tributary, Sand Lake Coulee. The estimated overbank sedimentation rate for the entire marsh is ~3,000 m 3 yr ‒1 for the recent period 1994–2006. In spite of reduced surface runoff and soil erosion in recent years, this recent sedimentation rate still exceeds by ~4 times the early settlement (1846–1885) rate of 700 m 3 yr ‒1 , when anthropogenic acceleration of upland surface runoff and soil erosion was beginning. The highest rate of historical bottomland sedimentation occurred from 1919 to 1936, when the estimated overbank sedimentation rate was 20,400 m 3 yr ‒1 . This rate exceeded by nearly 30 times the 1846–1886 rate. Artificial levees were constructed along the upper reach of Halfway Creek in the marsh during the early twentieth century to restrict flooding on the adjacent bottomlands. Anomalously high overbank sedimentation rates subsequently occurred on the floodplain between the levees, which also facilitated more efficient transport of sediment into the lower marsh bottomland. Although overbank sedimentation rates dropped after 1936, corresponding to the widespread adoption of soil-conservation and agricultural best-management practices, the continuation of anomalously high overbank sedimentation between the levees led to increased bank heights and development of a relatively deep channel. The deep cross-section morphology is commonly mistaken as evidence of channel incision; however, this morphology actually resulted from excessive overbank sedimentation. The historical metamorphosis of the Halfway Creek channel and riparian wetlands underscores the importance of understanding the long-term history of channel and floodplain evolution when restoration of channels and riparian wetlands are under consideration. Sedimentation patterns and channel morphology for Halfway Creek Marsh probably are representative of other anthropogenically altered riparian wetlands in the Upper Mississippi River System and similar landscapes elsewhere.

Abstract Neogene drainage development in southeastern Idaho has been influenced by drainage capture, Basin and Range faulting, volcanism, and the Late Pleistocene Lake Bonneville overflow and Bonneville Flood. In Marsh Valley, the Middle to Late Pleistocene sedimentary sequence is dominated by alternating lacustrine/paludal and alluvial sediments, which have yielded new 40Ar/39Ar, amino acid racemization, and luminescence age estimates. The pattern of sedimentation through time indicates poor drainage integration of southern Marsh Valley through most of the last ca. 640 ka and suggests slow basin subsidence along Quaternary faults mapped on the basin edges. Marsh Valley initially incised into that valley fill sequence ca. 19 ka, shortly before the Bonneville Flood. Marsh Creek is a markedly underfit stream occupying a meandering, broad valley carved into the valley fill sequence. These geomorphic and sedimentologic patterns suggest non-catastrophic Lake Bonneville overflow before and after the Bonneville Flood. In Portneuf Valley, ca. 8.5–7.4 Ma basin fill and a bedrock pediment are perched 800 m above the modern valley floor. Major incision of basin fill and bedrock by the ancestral Portneuf drainage system occurred prior to the Middle to Late Pleistocene, when two cut-fill events resulted in accumulation of alluvial fan deposits extending ~10–60 m above the modern valley floor and basalt extending ~10 m below to 20 m above the modern valley floor. Final incision by Lake Bonneville overflow is evident but relatively minor in comparison to the cumulative downcutting. Overall, incision is attributed to isostatic subsidence of the eastern Snake River Plain, which served as base level for the Portneuf drainage system after passage of the Yellowstone hot spot in late Miocene time.

Abstract Geologic, geomorphic, and geophysical analyses of landforms, sediments, and geologic structures document the complex history of pluvial Lake Bonneville in northern Cache Valley, NE Great Basin, and shows that the outlet of Lake Bonneville shifted ~20 km south after the Bonneville flood. The Riverdale normal fault offsets Bonneville deposits, but not younger Provo deposits ~25 km southeast of Zenda, Idaho. Rapid changes in water level may have induced slip on the Riverdale fault shortly before, during, or after the Bonneville flood. Although other processes may have played a role, seismicity might have been the main cause of the Bonneville flood. The outlet of Lake Bonneville shifted south from Zenda first 11, then another 12 km, during the Provo occupation. The subsequent Holocene establishment of the drainage divide at Red Rock Pass, south of Zenda, resulted from an alluvial fan damming the north-sloping valley. Weak Neogene sediments formed sills for the three overflowing stages of the lake, including the pre-flood highstand. Field trip stops on flood-modified landslide deposits overlook two outflow channels, examine and discuss the conglomerate-bearing sedimentary deposits that formed the dam of Lake Bonne ville, sapping-related landforms, and the Holocene alluvial fan that produced the modern drainage divide at Red Rock Pass. The flood scoured ~25 km of Cache and Marsh Valleys, initiated modest-sized landslides, and cut a channel north of a new sill near Swan Lake. Lake Bonneville dropped ~100 m and stablilized south of this sill at the main, higher ~4775 ± 10 ft (1456 ± 3 m) Provo shoreline. Later Lake Bonneville briefly stabilized at a lower ~4745 ± 10 ft (1447 ± 3 m) Provo sill, near Clifton, Idaho, 12 km farther south. An abandoned meandering riverbed in Round Valley, Idaho, shows major flow of the large Bonneville River northward from the Clifton sill. Field trip stops at both sills and overlooking the meander belt examine some of the field evidence for these shorelines and sills. The Bear River, which enters Cache Valley at the mouth of Oneida Narrows, 17 km ENE of the Clifton sill, was the main source of water in Lake Bonneville. It produced 3 sets of deltas in Cache Valley—a major delta during the Bonneville highstand, a larger composite delta during occupation of two Provo shorelines, and at least one smaller delta during recession from the Provo shoreline. The Bonneville delta and most of the Provo delta of the Bear River were subaqueous in Cache Valley, based on their topsets being lower than the coeval shorelines. The Bonneville delta is deeply dissected by closely spaced gullies that formed immediately after the Bonneville flood. The delta morphologies change sequentially from river-dominated to wave-dominated, then back to river-dominated. These unique shapes and the brief, intense erosion of the Bonneville delta record temporal changes in wave energy, erosion, vegetation, and/or storminess, at the end of the Pleistocene. Stops on a delta near Weston, Idaho, reveal many of the distinguishing features of the much larger deltas of the Bear River in a smaller, more concentrated form. We will see and discuss the ubiquitous gully erosion in Bonneville landforms, the nearly undissected Provo delta, the subaqueous topset of the Provo delta, and the wave-cut and wave-built benches and notches at the upper and lower Provo shorelines.

Abstract The area described herein is in southeastern Idaho, in Franklin and Bannock counties. It is readily reached via I–15 and various paved state highways from any direction. Each stop is easily accessible by two-wheel drive vehicles when roads are dry. However, the dirt roads leading to stops 1 to 6 can be extremely slick when soaked. Permission from property owners is required if you desire to wander beyond each stop. The tour is keyed to the following 7½-minute quadrangles: Pocatello North, Inkom, Arimo, McCammon, Bonneville Peak, Downey West, Downey East, Oxford, and Swan Lake. The latter two are particularly important for locating features south of the overflow site. The Preston and Pocatello 2-degree quadrangles are also suggested. The trip will take about six hours. It begins in northern Cache Valley at the hamlet of Oxford, proceeds northward through Red Rock Pass and Marsh Valley, and ends in the Pocatello area (Fig. 1). Food and lodging can be found in Preston (dry) and Pocatello.

Abstract Detailed stratigraphic study, paleoenvironmental interpretation of tidal wetland facies based on macroflora and agglutinated foraminiferal assemblages, radiocarbon dating, and modern marsh accretion rates are used to reconstruct the late Holocene sea-level history of the Leipsic River valley. Transgressive valley-fill deposits of the Leipsic River valley consist of brown peat, olive-gray mud, and gray-brown muddy peat. These facies were deposited in brackish wetland environments, open-water subtidal environments, and modern salt-marsh environments, respectively. In the early Holocene the Leipsic River was a tributary to a major fluvial system, possibly the paleo–Delaware River. The Holocene transgression reached the area about 5,000 yr BP, when fringing tidal wetlands began to develop in both valleys, depositing brown peat. Rapidly rising sea level flooded the valley of the Leipsic River by 3,000 BP, turning it into an open-water estuarine environment. After 3,000 yr BP, the rate of the sea-level rise decreased, resulting in wide expansion of brackish wetlands in the Leipsic River valley and along the Delaware Bay coast. Tidal creeks migrating on the marsh paleosurface were eroding brown peat and depositing mud units at different depths. The brackish conditions persisted in the area until about 1,000 yr BP. One thousand years ago a change in the environments occurred when modern salt marshes began to replace the brackish wetlands. Sea level was approximately 12 m below modern MHW when the first emergent tidal wetlands were developed in the valley. By 4000 yr BP, rapidly rising sea level reached an elevation of 9 m below MHW. From 4000 to 2000 yr BP, sea level rose to an elevation of 3.5 m, and by 1000 yr BP it reached an elevation of about 3 m below the modern marsh surface. Salt marshes developed in the valley during the last 100 years with a vertical marsh accretion rate of 0.29 cm/yr. Twenty years ago, the marsh vertical accretion rate increased up to 0.46 cm/yr. These rates are comparable to the average rate of sea-level rise of 0.33 cm/yr measured by the tide gauge at Breakwater Harbor, Delaware. Thus, the salt marshes at Bombay Hook National Wildlife Refuge are in a dynamic equilibrium with rising sea level.