Last Wednesday AZGS geologists Phil Pearthree and Ann Youberg went to Lake Roosevelt to look at the "Orange Peel Mudflow" [left photo, credit Ann Youberg]
Ann shared her notes from their assessment:
The sediment-laden flow occurred on Sept 21 in an unnamed drainage that enters Lake Roosevelt at the Orange Peel Rec Area turnoff at the north end of the lake. The sediment plugged the box culverts at Highway 188 and a fan filled the channel and flowed over the road 2+m thick [note Phil Pearthree for scale next to exposed flow. Photo credit Ann Youberg] . This area was burned by the 2005 Edge Complex. We wanted to see what kind of flow occurred, and where the sediment came from. Three to seven years after a wildfire is typically the period of lowest root strength, so we wanted to see if there were any landslides that contributed to the sediment input.
The flow was not a flood flow, but we can't say with certainty whether it was a hyperconcentrated flow or a debris flow. The flow was composed of grus ranging in size from sand - med pebble. We did not observe any sorting or bedding, but there wasn't a large range in material sizes. We did see bedding in the older bank sediments, but not in the deposits of this flow. We also saw lots of chunks of bank that had fallen onto the flow but not incorporated into the flow, suggesting they were rafted.
The channel was surprisingly steep - Phil's calculations ranged from 9-11%. In the upper watershed we saw lots of rilling and gullying on the hillslopes, but we did not see any evidence of mass movement. We assumed the rilling and gullying was post-fire but when I looked at the pre-fire 2005 NAIP imagery the hillslopes were rilled and gullied then too. It is just a very erodible landscape, which is probably why there was so much sediment available for transport.