We present taphonomic analyses of the Standing Rock Hadrosaur Site (SRHS), a vast Edmontosaurus annectens bonebed in the Maastrichtian Hell Creek Formation of South Dakota, which yields important insights into hadrosaurid paleobiology and environmental settings recorded by basal Hell Creek strata. Though Edmontosaurus bonebeds have been described from other Late Cretaceous formations in the Western Interior, namely the Lance, Prince Creek, and Horseshoe Canyon formations, our study provides the first thorough description of an Edmontosaurus bonebed from the Hell Creek Formation. SRHS is also the first formally described bonebed of E. annectens. Taphonomically, representation of every skeletal element, horizontality of most bones, and rarity of weathering and abrasion suggest brief preburial exposure and transport with minimal sorting bias. Near-universal disarticulation and disassociation, localized orientation of bones, and infrequent preburial breakage indicate moderate flow energy during deposition. Additional fauna, though rare, are indicative of a fluvial-coastal setting, and palynofloral analyses signify deposition in a small, shallow floodplain lake surrounded by cypress forests. Cumulatively, these data indicate that a herd of primarily subadult and adult Edmontosaurus died in a nearby fluvial setting in a mass mortality event and, following brief decay and scavenging by theropods, their bones were buried in a shallow floodplain pond by a flooding event/crevasse splay. Our findings provide supporting evidence for the hypotheses of gregarious herding behavior in hadrosaurids and age structuring of Edmontosaurus herds.