Historical Analysis of 24-Hour Storms in the Upper Midwest

Andy Erickson; Noah Gallagher; John Gulliver

Extreme precipitation estimates, particularly for rare storms with large return periods like the 100-year storm, are a useful tool for watershed managers and engineers to evaluate the flood risk for their communities. While national level estimates such as Technical Paper 40 (Hershfield, 1961) and Atlas 14 (Perica et al., 2013) are publicly available, these estimates do not consider temporal dynamics (i.e., non-stationarity) of extreme precipitation. Identifying any temporal trends at a single weather station can be challenging because most modern observation records extend fewer than 150 years, which is insufficient to run statistical tests or identify trends over time for rare events. This presentation examines the evidence for both long term trends in extreme precipitation frequency, and any breakpoints in such trends using long term precipitation records from over 1,000 stations in the Midwest USA. First, we identify non-stationarity in the aggregate frequency of occurrence of top 50% and top 10% annual maximum precipitation events, as well as in the occurrence of largest recorded event at each station. Second, our results show that a piecewise linear fit with two inflection points occurring during the years 1920 and 1974 accurately models observed extreme event non-stationarity. The Dustbowl period in the American Midwest explains the 1920 breakpoint and the 1974 breakpoint coincides with surface temperature increases for the continental United States. We will discuss these findings.

Uncertainty Monsters and Nonstationary Flood-Frequency Analysis

Karen Ryberg

Flood-frequency analysis is undergoing a paradigm shift. Traditionally grounded in stationary assumptions, the field now faces growing pressure to adopt nonstationary methods that account for climate and land-use change. This shift is not just technical—it is philosophical, methodological, and deeply uncertain. Drawing on consulting experience, I examine how practitioners navigate these uncertainties using the “uncertainty monster” metaphor, which captures the tension between competing analytical frameworks. Originally developed in the Netherlands, the “uncertainty monster” metaphor describes situations where mutually exclusive categories, knowledge and ignorance, facts and values, prediction and speculation, stationary and nonstationary, coexist and create tension. In flood-frequency analysis, the monster emerges in the choice between stationary and nonstationary assumptions, each with distinct methodological and interpretive challenges (more monsters). Six strategies for dealing with uncertainty monsters have been identified: denial, exorcism, adaptation, assimilation, embracement, and anesthesia. These are not just theoretical—they are visible in practice. Denial persists in exclusive reliance on Bulletin 17C; exorcism in efforts to eliminate uncertainty through complex models; adaptation in scenario-based approaches; assimilation in rethinking foundational categories; embracement in celebrating complexity; and anesthesia in consensus-driven decisions. The role of consultants is to reduce the monster—not by denying or exorcising it, but by translating complexity into actionable insight. This presentation draws on case studies to illustrate how adaptation and assimilation strategies can be utilized in flood-frequency analysis.

Upper Mississippi River Restoration (UMRR) Lower Pool 10 and McGregor Lake Habitat Rehabilitation and Enhancement Projects (HREP)

Angly Ulmschmid, Riley Mondloch, and Mark Christenson

The Lower Pool 10 (LP10) HREP is located on the Iowa side of the Upper Mississippi River in Pool 10, near Guttenberg, Iowa; McGregor Lake is a 200-acre backwater lake in Pool 10 near Prairie du Chien, Wisconsin. Both sites lie within the Upper Mississippi River National Wildlife and Fish Refuge. The purpose of the HREPs is the rehabilitation of selected river segments to address the degradation of habitat quality as a result of impoundment, island loss, high hydrologic connectivity, low diversity in riverbed depth and configuration, and the decline of vegetation over the past decades. Project features include the construction of new islands with a base of granular fill, obtained from the dredging of the navigation channel, capped with fine-grained material obtained from the dredging of overwintering habitat features, as well as the construction of hydraulic rock structures. The permanent features will restore the riparian border between the secondary channels and adjacent backwaters, repairing and enhancing the existing hydro-geomorphic conditions which in turn will increase the sediment transport capacity in the channels enhancing natural land-building processes along channel borders. Newly constructed and rehabilitated islands and rock structures will restore bathymetric diversity and seasonal water level variation which will benefit, enhance, and maintain a resilient mix of riverine, backwater and floodplain habitat. The restored habitat includes shallow lentic habitat, semi-lotic wetland habitat, newly created and restored island habitat, and deep-water fish overwintering habitat. Project design and construction is 100% federally funded, with an estimated value of $39M, LP10 is the largest UMRR project to ever proceed to construction. With a total of $24M all construction is completed at McGregor Lake as of July 2026.