It is valuable to construct likelihood functions that rigorously incorporate measurement errors and annual peak discharge, historical, and paleohydrologic bound information in Bayesian flood frequency analyses. Estimates of primary posterior modes for common three‐parameter frequency distributions are constructed using simulated annealing and the simplex method. Parameter and flood frequency probability intervals are calculated directly by systematic parameter space integration. Bayesian flood frequency analyses with annual peak discharge, historical, and paleohydrologic bound data for the Santa Ynez River, California, and the Big Lost River, Idaho, demonstrate that paleohydrologic bounds reduce quantile biases by placing large observed peak discharges in their proper long‐term contexts and substantially narrow peak discharge confidence intervals when estimating floods with low exceedance probabilities.

Stratigraphic and geomorphic evidence along the Big Lost River at the Idaho National Engineering and Environmental Laboratory (INEEL) defines age and paleostage limits for a paleoflood ϳ400 yr ago with an estimated discharge of ϳ100 m 3 /s. The discharge for this paleoflood is ϳ40% larger than the flood of record from a gaging site near Arco where flow is regulated, but is smaller than 6 historical peak discharges from a gage in the unregulated upstream portion of the drainage basin. The paleoflood is the largest flood along the Big Lost River in the past ϳ400 yr and confirms that large downstream decreases in Big Lost River peak discharge predate historical stream diversion and regulation. Flow simulations indicate that discharges only slightly larger than ϳ110 m 3 /s will initiate extensive flow across the unmodified Pleistocene alluvial surfaces that flank the Big Lost River on the INEEL site. The geomorphology of these surfaces and two-dimensional flow simulations are the bases for establishing a paleohydrologic bound at a discharge of 150 m 3 /s for the past 10 k.y.When the paleoflood and paleohydrologic bound data are included in peakdischarge-frequency analyses, they provide strong constraints on peak discharge for annual probabilities from Ͼ10 ‫2‬ to 5 ‫‬ 10 ‫5‬ . Sensitivity testing is used to assess the potential impacts of historical regulation of annual peak discharge and of alternative characterizations of the paleohydrologic information on discharge-frequency estimates. These tests demonstrate that for annual probabilities of 10 ‫2‬ and 10 ‫4‬ , the upper limits of peak discharge are unlikely to exceed ϳ110 m 3 /s and ϳ170 m 3 /s, respectively, as long as the long-duration paleohydrologic bounds are included in the analyses. In contrast, peak-discharge-frequency analyses using only annual peakdischarge discharge data result in estimates that range from ϳ105 m 3 /s to Ͼ170 m 3 /s for an annual probability of 10 ‫2‬ . Adding paleohydrologic information to dischargefrequency analyses reduces the possible range of discharge estimates over a wide range of annual probabilities.

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, ∼6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4–4.6 ka) slip rate of ∼1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4–9.9 ka) slip rate exceeds the Holocene (9.9–4.6 ka) rate by a factor of ∼2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The ∼5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of ∼4–5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles.

No abstract

No abstract