Flood of June 2008

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Community Maps and Data

• • Reedsburg
  • Rock Springs
  • La Farge
  • Gays Mills
  • Milford
  • Jefferson
  • Fort Atkinson
  • Janesville
  • Beloit

Estimated Peak Streamflows (Excerpt from SIR 2008-5235)

Peak-gage-height data, peak-streamflow data, and estimated flood probabilities from the June flood for 32 USGS streamgages and 3 ungaged locations are presented in tables 3 and 4. The data listed in table 3 have not received final checks as of the date of writing (December 2008) and are considered provisional until published in the USGS “Water-Resources Data for the United States” annual report for water year 2008. Table 5 lists the correspondence between flood probability and recurrence intervals for commonly used flood probabilities. New gage-height or streamflow records were set at 21 USGS streamgages. Flood probabilities at the streamgages with record gage-height or streamflow ranged from 0.002 to 0.04 (range based on 95-percent confidence intervals). Five streamgages had estimated flood probability ranges of 0.005 or less based on the 95-percent confidence intervals (table 3). The Baraboo River peaked at 10 ft above flood stage (National Weather Service, 2008). Some streams rose and fell rapidly beginning on June 7 or soon after (table 3, fig. 5). Large streams took longer to peak; the Rock River at Afton did not peak until June 21, and flooding continued into July (table 3, fig. 5).

The June 2008 flood took on different characteristics in each of the nine severely damaged communities included in this study. At Reedsburg and Rock Springs, flows of the Baraboo River peaked at approximately 11,500 to 12,900 ft3/s in the early hours of June 10 after especially heavy rainfall fell on the watershed upstream from these communities (table 4). Farther downstream and to the east, the Baraboo River at Baraboo peaked 3 days later from additional thunderstorms that hit the eastern part of the watershed harder than the western part upstream from Reedsburg (figs. 1 and 5). The flooding on the Baraboo River likely had flood probabilities of 0.002 or less (table 3). Flooding on the Baraboo River was responsible for unprecedented closures of Interstates 90-94 and 39 north of Madison.

The Kickapoo River at the communities of La Farge, Gays Mills, and Steuben rose and fell rapidly from June 8 to June 12 following the weekend rains on June 7–8 (fig. 5; tables 3 and 4). Peak streamflows at La Farge, Gays Mills, and Steuben had flood probabilities of 0.002 to 0.01 (tables 3, 4, and 5). Peak streamflow for Gays Mills was estimated to range from 19,200 ft3/s (rating extrapolation) to 22,000 ft3/s (step-backwater model, Robert Watson, National Wisconsin Department of Natural Resources, written commun., 2008). Peak streamflow estimates for the Kickapoo River at Gays Mills seem low compared with peak streamflows for La Farge (upstream 22,100 ft3/s) and Steuben (downstream, 28,700 ft3/s); however, comparison of historical peak streamflows of the Kickapoo River from the three streamgages indicate that the peak streamflows can vary from what would be expected based on basin size. Possibly storm tracks and flood-plain storage, among other unknown factors, affect peak streamflow relations among the three streamgages. Gage heights for the June 2008 flood peak in La Farge, Gays Mills, and Steuben were 0.9, 1.9, and 4.4 ft higher than the gage heights for the July 1978 flood, respectively (fig. 6, tables 3 and 4; Hughes and others, 1981). For Gays Mills, the June 2008 gage height was approximately 0.3 ft higher than the August 2007 flood-peak gage height (fig. 6).

Farther east, the Crawfish River at Milford rose more slowly than the Kickapoo and Baraboo Rivers and peaked on June 16 after the second set of thunderstorms on June 12 (fig. 5, table 3). The low-lying gentle topography and numerous wetlands in the Crawfish River watershed contributed to the long duration of the flood, which lingered into the latter part of June. Peak streamflow for the Crawfish River likely had a flood probability less than 0.01 (table 3).

The communities of Jefferson, Fort Atkinson, Janesville, and Beloit along the Rock River also experienced prolonged flooding (fig. 5), and the Rock River did not peak until June 21 at Indianford and Afton (table 3). Estimates for flood probabilities for the flooding along the Rock River range widely, from 0.002 to 0.04 (table 3). The extended time for the Rock and Crawfish Rivers flooding resulted in the unprecedented closure of westbound Interstate 94 for June 13–19 between Milwaukee and Madison (Channel 3000, 2008).

Figure5. Hydrographs showing selected USGS streamgages in southern Wisconsin for June-July 2008.

Table 3. Provisional flood-peak gage heights, peak streamflows, and estimated flood probabilities during the June 2008 flood at selected U.S. Geological Survey streamgages in southern Wisconsin.

[The data shown are considered provisional as of the date of publication. Final data will be published in the USGS "Water Resources Data for the United States" annual report for water year 2008. See figure 3 for streamgage locations. Peak of record shown in bold. Estimated flood probability range based on estimates: The flood probability for a particular streamflow is the probability or odds of that streamflow being equaled or exceeded in any given year. For example, a probability of 0.01 means there is a 1 percent chance of that flow magnitude being equaled or exceeded in any given year. Stated another way, the odds are 1 in 100 flow will equal or exceeded that magnitude in any given year. The traditional concept of recurrence interval is directly related to the flood probability. By definition, the recurrence interval corresponding to a particular flood probability is equal to one divided by the flood probability. For example, the flood probability of 0.01 corresponds to the 100-year flood. Based on direct comparison of flood-peak discharge to the discharge estimates for various flood probabilities (traditional approach). Estimated flood probability range based on 95-percent confidence intervals: Based on comparison of flood-peak discharge to the lower and upper bounds of the 95-percent confidence limits for discharge estimates for various flood probabilities (alternative approach). Abbreviations: mi², square miles; ft³/s, cubic feet per second, yr, year; >, greater than; < less than; n/a not applicable or not available; ND, not determined; e, estimated; P, present; LP3, procedure for calculating flood probabilities by fitting systematic annual-peak-discharge data to a log-Pearson type III distribution (Interagency Advisory Committee on Water Data, 1982). WIE, flood probabilities calculated using weighting of independent estimates procedure (Interagency Advisory Committee on Water Data, 1982, appendix 8); GIS, used GIS-based basin characteristics for regional regression equations used in WIE procedure; Regional regression equations from Walker and Krug (2003)]

Table 4. Estimated peak gage heights, streamflows, and flood probabilities for selected locations in southern Wisconsin, June 2008 flood.

[Estimated flood probabilities based on regional regression equations from Walker and Krug (2003). Peak flow for given flood probability: The flood probability for a particular streamflow is the probability or odds of that streamflow being equaled or exceeded in any given year. For example, a probability of 0.01 means there is a 1 percent chance of that flow magnitude being equaled or exceeded in any given year. Stated another way, the odds are 1 in 100 flow will equal or exceeded that magnitude in any given year. The traditional concept of recurrence interval is directly related to the flood probability. By definition, the recurrence interval corresponding to a particular flood probability is equal to one divided by the flood probability. For example, the flood probability of 0.01 corresponds to the 100-year flood. Abbreviations: USGS, U.S. Geological Survey; mi², square miles; ft³/s, cubic feet per second; YR, year; <, less than; >, greater than; HWM, high-water mark; NWS National Weather Service; na, not available]