as G. K. Gilbert (1890), that Lake Bonneville drained catastrophically and lost 105 meters depth of water through spillover erosion at Red Rock Pass, Idaho. However, as shown by Adams and Bills (2016), Lake Bonneville basin’s interior topography has been significantly elevated by post-lake isostatic rebound. Adams and Bills (2016, p. 160) revise O’Connor’s volume estimate of the Lake Bonneville Flood significantly upward to 5135 cubic kilometers. Because Hopi Lake was deeper than Lake Bonneville, the isostatic rebound effect would be enhanced in the case of Hopi Lake. Therefore, we estimate, conservatively, Hopi Lake’s volume to be 5200 cubic kilometers (1250 cubic miles). Our assumption of complete drainage of Hopi Lake makes the Hopi Lake Flood (5200 cubic kilometers) slightly larger than the Lake Bonneville Flood (5135 cubic kilometers). The largest Lake Missoula Flood, from the Ice Age constriction of Clark Fork River in Montana, has been estimated at 2500 cubic kilometers (600 cubic miles) by O’Connor et al. (2021). Thus, the Hopi Lake Flood was slightly larger than the Lake Bonneville Flood, and more than twice the size of the biggest Lake Missoula Flood. These lakes and their erosive floods can be compared to modern Lake Michigan (4900 cubic kilometers). A very big question that needs to be discussed is the relationship of Bidahochi Basin and its Hopi Lake to upstream Utah and Colorado basins and their possible ancient lakes. Conventional, twentieth century geomorphic dogma since the time of William Morris Davis has asserted that streams erode upstream from lowland plains toward highland plateaus (Hilgendorf et al., 2020). That historically dominant viewpoint has been called “headward erosion” and has profoundly impacted geologist’s thinking about Grand Canyon erosion. An alternate interpretation is that highland basins quickly fill with sediment and water as topographic barriers overflow or spillover. This new model has revitalized John Newberry’s oldest 1858 thinking about Grand Canyon and promoted top-down, not bottom-up, drainage basin integration. Therefore, Bidahochi Basin and the putative Hopi Lake can be critical elements in Grand Canyon thinking. Also, of critical importance in the top-down spillover discussion, is whether any lakes upstream of Hopi Lake broke their topographic barriers and filled Hopi Lake before it broke the Kaibab Plateau barrier to erode Grand Canyon. Figure 21 depicts the configuration of lakes, including a big lake in Utah. Edmond Holroyd, employed for many years as a Bureau of Reclamation scientist, was one of the earliest to rediscover the power of “top-down” thinking. In 1986 when living in Colorado next to Black Canyon of the Gunnison River, Holroyd discussed the Gunnison River drainage question with the resident USGS geologist Dr. Wally Hansen, one of the architects of the headward erosion hypothesis (Austin et al., 2020). During the discussion with Hansen about Black Canyon, Holroyd adopted top-down thinking that Black Canyon was excavated by spillover drainage of a high lake. Holroyd postulated other lakes downstream of Gunnison. Late in 1986 he used a government DEM to plot “big lakes” which could form on modern Colorado Plateau landscape if Grand Canyon was blocked at the 5,700-foot elevation (Austin et al., 2020). The computer chart made from the DEM allowed Holroyd to postulate a “big Utah lake” (Holroyd, 1987; Holroyd, 1988; Holroyd, 1990; Holroyd, 1994). Field work in July 1987 allowed Holroyd to recognize that the big Utah lake breached its bedrock dam at Lees Ferry and drained catastrophically to erode Marble Canyon. That big Utah lake was later called Canyonlands Lake (Austin, 1994). Then, a priority dispute occurred about who first proposed “the big Utah lake” and its breaching at Lees Ferry to form Marble Canyon. Walter Brown (2008) asserted, we believe incorrectly, that he had priority after field work in summer 1988 and public presentations later in 1988. Brown published a lake map in 1989 (Brown, 1989). However, Holroyd appears to have priority over Brown because Holroyd’s lake map was circulated in 1987 and his written proposal (Holroyd, 1988) describing Lees Ferry breaching and Marble Canyon erosion was circulated in January 1988 (see Austin et al., 2020). Was there a precursor to Hopi Lake within the Bidahochi Basin? Sedimentary evidence in the lower Bidahochi Formation indicates that the basin was vacant of a deep lake (Austin et al., 2020; Douglass et al., 2020). Good evidence of Hopi Lake being very deep (~1860 meters elevation) is first found in the upper Bidahochi Formation. That evidence is the green, gastropod-bearing, fresh water clays of the upper Bidahochi Formation. Did Canyonlands Lake fail at Lees Ferry creating an abrupt addition of water to Bidahochi Basin quickly raising the level of Hopi Lake? That explanation would be in accord with the top-down perspective that has become consensus spillover thinking (Blackwelder, 1934; Meek, 2019; Helgendorf et al., 2020; Larson et al., 2022). Geological evidence of rapid filling of Hopi Lake is evident at the transgressive shoreline terraces at Wagon Box Draw Landform Tract. There the tufa is a very thin encrustation indicating that the lake did not endure at high level for a long time. Thus, we regard the transgressive shoreline depositional terraces at Wagon Box Draw to be evidence for rapid basin filling and abrupt disappearance of Hopi Lake. After Canyonlands Lake failed at Lees Ferry, the new basin configuration could allow the final filling of Hopi Lake to extend northward to Colorado. Therefore, an enormous temporary lake (much larger than 5200 cubic kilometers) could have resided directly east of Kaibab Plateau, and could have been ready to overtop Kaibab Plateau barrier, and erode Grand Canyon. We have not seen a regressive sequence of shoreline terraces for Hopi Lake that would be evidence for slow disappearance of Hopi Lake. Because we have not seen such a regressive shoreline sequence, we assume that highstanding Hopi Lake disappeared from its basin rapidly. That implies lake drainage by catastrophic spillover erosion. The same can be said for highstanding Lake Bonneville, that also lacks regressive shoreline terraces (Chen and Maloof, 2017; Oviatt, 2020). The catastrophic failure of Lake Bonneville’s dam at Red Rocks Pass in Idaho produced the enormous flood on the Snake River Plain (O’Connor et al., 2021). CONCLUSION We interpret these tufa-encrusted landforms at Wagon Box Draw to be transgressive shoreline terraces carved within the Kaibab Limestone slope as Hopi Lake rose quickly to fill Bidahochi Basin. These shoreline landforms indicate a basin-filling, deep lake. We point to several similarities with the high shoreline terraces of Lake Bonneville. Using the consensus model for high shorelines of Lake Bonneville, our model specifies how the terrace is first eroded into thin-bedded limestone and then is later deposited with residual gravel. Our model specifies an erosional platform and landward erosional scarp are inscribed, and then, because of transgression, the platform and scarp are buried by residual coarse gravel. Finally, because of continued quick transgression, the shoreline depositional terrace is accreted with a thin crust of tufa. We believe that filling of Bidahochi Basin was accelerated by breaching of Canyonlands Lake upstream in Utah. Top-down overflow of higher basins promoted quick filling of Hopi Lake, initiated catastrophic spillover erosion of Grand Canyon, and caused rapid drainage of Hopi Lake. AUSTIN, HOLROYD, FOLKS, AND LOPER Shoreline Transgressive Terraces 2023 ICC 360
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