On a geologic timescale, the Willamette Valley has been the site of massive floods. During the late Wisconsin glaciation, a series of floods (known as the Missoula or Bretz Floods) occurred. The last flood in the series, a massive flood with an estimated of water flowing at a rate of 42 km3 per hour (412 million ft3 per second) over a 40-hour period, occurred about 13,000 years ago. The flood filled the Willamette Valley to a depth of about , in a near-perfect overlay of the camas pocket gopher's range. Although the species has been collected above this elevation, such finds are uncommon. A temporary lake, Lake Allison, formed. Although it is assumed that the gopher lived in the valley before the flood, no fossils have been recovered. The Chehalem Mountains, with a peak elevation of , probably provided refuge for survivor populations and survivors would have repopulated in isolated pockets when the waters receded. Before and since the floods, the mountains are thought to have limited gene flow between populations. The relatively narrow, sluggish Willamette River does not appear to obstruct genetic flow in gopher populations.