The flow has clearly come out of a deeply incised gully in the mountains:
A perspective view of the excellent Google Earth imagery of this area is rather helpful:
Two things to note here (the flow hit the area just above the "97" on the image above). First the community that has been hit by this flow is located on a small fan that has been formed by this type of flow. Second, the very narrow, deeply-incised channel mouth is the classic location for this type pf flow, where a blockage allows debris and water to accumulate, before collapsing to generate a flow. It is interesting to note that this area has reportedly experienced unusually heavy rainfall this spring and early summer.
Some of the news reports are indicating there may mave been a small dam or culvert failure at the dam in the upper watershed. Not yet confirmed by geologist as the debris flood took place late in the day Sunday.
ReplyDeleteNot a lot of regard for the risk of debris floods in this area on U.S. side of the border regarding siting of structures and this debris flow would indicate a similar under appreciation on the Canadian side. I suspect that most alluvial fan deposits in this area are associated with intense summer thunderstorms.
There is (was) a small dam at the top of the creek. You can see it in Google Maps if you follow the creek up. Its outlet was a culvert under a road. The dam was evidently built decades ago for irrigation, and lies within the boundary of a more recently established Provincial Protected Area.
ReplyDeleteAs far as the weather goes, it has been a cool spring with lots of rain, and it got significantly hotter over the weekend than it has been. The snowpack was above average, and I would not be surprised if it is still melting up there.