Tuesday, September 29, 2009

Extent of lake caused by the Greeley Bar lava dam

I created this lake by generating a contour from the LiDAR dataset at an elevation of 1046 m. GlobalMapper does this in about 1.5 minutes. Then, exported the vector as a shapefile, cut out the parts of the line that occur downstream from the dam, stitch the remaining loose ends, build a poly from the line and there it is.

This lake has an interesting topographic correspondence with the old landslides on the south side of the Hole in the Ground as well as the ancient fan remnants that come in from the north side. Don't forget that much of the topography you can see through the lake didn't exist at the time of the lava dam. The valley floor was probably formed on the Bogus Rim lava which forms the flat-topped features that flank the left and right banks of the river near the eastern end of the lake. The top of the Bogus Rim lava is only about 25 m below the surface of this lake. Thus, the link between this lake and the landslides is dubious as there was nowhere for the landslides to slide.

Posted via email from Fresh Geologic Froth


  1. Dr. Jerque,

    Just to discuss a little: You say above that the valley floor at the time of the GB lake was probably at the elev of the BR lava but that the landslides probably weren't there (b/c the BR lava was there and) b/c there was no space for them to slide into. But how do we really know that? All we know about the age of the GB lava is >780k. So for fun's sake say the GB lava is 1Ma, that leaves a long time in between the GB and the ~1.9 Ma BR lava for erosion to remove a lot of the BR. Perhaps much of the BR lava was gone by the time of GB lava? What field evidence do we have that the GB and BR are similar in age--perhaps a lack of extensive sedimentary deposits intercalated between them?

    Also, (as you noted) I think it is very interesting how the hanging alluvial fan on the north side of the valley adjacent to Juniper Canyon is above the waterline of the BR lake as you have it at 1046m. Why? Is that b/c the fan was graded to the elev of the GB lake or the BR lava?

    Why did you choose 1046m (3430ft) for the max dam height? How about 3460ft (a candidate max elev of the GB lava on the unnamed cascade)?


  2. The point is to discuss it alot! Thanks for reading and commenting. Admittedly, I was so enamored with my relatively quick way to construct the lake that I didn't give all of these issues due consideration. As for the assumption about the lack of topography, I suppose that is driven by the juxtaposition of the two lavas with no obvious intervening sediments, the position of the lava delta, and the downstream distribution of the two lavas. If there were a particularly deep gorge developed between the timing of the Bogus Rim and the Greeley Bar, then the GB would have filled that gorge completely and flowed up and over the top of the BR in the narrows downstream. If that happened, how big could the gorge have been to have accommodate the influx of lava and allowed it to spill over the sides, so to speak? Could we find some textures in the lava that would support that? Check the gigapan of the Deer Creek site and think about it. Seriously...I'm not disputing the concept, I just have trouble visualizing it.

    As for the dam height, I only went with my best estimate of the top of the lava delta and not the high point on the lava...I will do that for comparison.

    As for the fan, I recall no evidence that the sediments were deposited in a lake. I think the correspondence is a coincidence. Also, the correspondence will be less striking with a deeper lake/higher dam. A comparable fan toes out on the small Bogus Rim remnant about a mile to the southwest of the larger fan remnant. I suspect that is a correlative unit that speaks for the locally stable baselevel provided by the Bogus Rim lava.

    We really need a good age on the Greeley, no?

