• Friends: Please double-check the email address you have on file. Make sure that it is current and able to receive email. When our emails are rejected it can damage our ratings and slow down future deliveries.
    Thanks!

New report from Returning Rapids Project

drewsxmi

Well-Known Member
'The main reason is that they would likely get blocked by sediment somewhat early in the life of the reservoir ...'

Floyd Dominy thought the sediment build-up could be a problem sometime around the year 2960.

Are you calling him out as a liar, drew?
He was working from more limited information than we have now, and may have firmly believed what he said. I've stated earlier in this thread that the raw numbers have the reservoir filling up with sediment in about 700 years, but that's the entire reservoir filling up. He and David Brower reportedly got into a shouting match about how fast the reservoir would get filled by sediment.

Sediment build-up is already causing problems in the San Juan Arm (raising the river grade at Clay Hills Crossing), and the sediment delta from the San Juan Arm will reach the main channel confluence before the sediment delta in the main channel reaches the confluence. It's already eliminated the use of the Hite Marina for lower water levels, and would/will soon eliminate the use of Hite Marina for even full pool levels if the water stays at that same level for longer.

I think way the sediment distributes as it fills in will be much more of an issue than the total time.
 

Dougie

Well-Known Member
More on Dominy from Wikipedia. If this is true, I would like to have been a “fly on the Coleman Stove” with Brower and Dominy in close quarters on the same raft for a few days.

Dominy was a main character in two non-fiction books about water management the American west: Cadillac Desert by Marc Reisner and Encounters with the Archdruid by John McPhee. McPhee arranged a whitewater rafting trip down the Colorado River with Dominy and David Brower, a prominent environmentalist and founder of Friends of the Earth, and the book highlights their opposing views of the river and its uses.
 

bob london

Active Member
More on Dominy from Wikipedia. If this is true, I would like to have been a “fly on the Coleman Stove” with Brower and Dominy in close quarters on the same raft for a few days.

Dominy was a main character in two non-fiction books about water management the American west: Cadillac Desert by Marc Reisner and Encounters with the Archdruid by John McPhee. McPhee arranged a whitewater rafting trip down the Colorado River with Dominy and David Brower, a prominent environmentalist and founder of Friends of the Earth, and the book highlights their opposing views of the river and its uses.

Katie Lee (RIP) wanted someome else to cut off Floyd's (RIP) bollocks. If Lee and Dominy were forced together for a two week float down The River, one of them would have taken out the boat battered and bruised. Katie would have shrugged her shoulders and blamed his injuries on the 'savage river that must be tamed'.
 

The Oracle

Active Member
Bob,
Yup. Deadpool is indeed 3370'. To be honest I don't know why they don't have lower outlets on dams than that but they don't. Perhaps someone proficient in engineering can help explain why they don't build them with river level outlets and such. Also what's "offensive" is absolutely nothing haha. It's just a way of poking fun at certain people who take things too seriously sometimes.
For big (relatively deep) dams, it would be too much pressure (head), volume, and flow velocity to economically and safely control. So, the 'concerns' would be; 1) erosion of valve assembly; 2) built-in weak/failure point; 3) inability to repair/maintain with acceptable risk. For instance, a 300' head results in about 150 PSI for us laymen. Having a valve large enough in diameter to reduce erosional velocity through the valve whilst providing adequate flow volume would result in valve several yards in diameter. Even if it is needle type valve, it would be ENORMOUS and TERRIFYING to have sitting there. I reckon 80+% of the game, though, is nobody ever figured these lakes would get low enough to worry about warranting having one...lol.
 

jayfromtexas

Well-Known Member
For big (relatively deep) dams, it would be too much pressure (head), volume, and flow velocity to economically and safely control. So, the 'concerns' would be; 1) erosion of valve assembly; 2) built-in weak/failure point; 3) inability to repair/maintain with acceptable risk. For instance, a 300' head results in about 150 PSI for us laymen. Having a valve large enough in diameter to reduce erosional velocity through the valve whilst providing adequate flow volume would result in valve several yards in diameter. Even if it is needle type valve, it would be ENORMOUS and TERRIFYING to have sitting there. I reckon 80+% of the game, though, is nobody ever figured these lakes would get low enough to worry about warranting having one...lol.
That wouldn't surprise me. I don't think anyone ever imagined that Lake Powell would be in such dire straits, but so many years of poor inflows in a 20 year period can do that.
 

dgoodwin

New Member
Yup. 3490' is indeed minimum power pool. The next elevation below to be concerned about is 3370, which is dead pool. Below that the dam cannot pass any water downstream to Lake Mead.
So, let me wrap my head around this. If (when) Lake Powell gets to an elevation of 3,370-or below, it will not be possible for water to flow out to create the Colorado River through Grand Canyon? Not considering power generation- is there no way to release water through the dam once it gets below that level?
 

drewsxmi

Well-Known Member
So, let me wrap my head around this. If (when) Lake Powell gets to an elevation of 3,370-or below, it will not be possible for water to flow out to create the Colorado River through Grand Canyon? Not considering power generation- is there no way to release water through the dam once it gets below that level?
Well it can't really get below that level. Assuming that they left the outlet works open all of the way, the lake level would go up and down a little bit to push sufficient water through the outlet works to balance the inflow, just like any natural lake with an outlet. If they wanted to release more water through the dam below 3,370' elevation they would need to change something at the dam, like boring new bypass tunnels. (All of this assumes that there is not some secret design feature that would allow water releases below 3,370'.) There is very little stored water left at that level, somewhere between 700,000 af (water-data.com) and 1,700,000 af (1986 sedimentation survey), versus 24,322,000 full. The cost of bypassing the dam, the possibility of putting the dam back in commission, and so forth would need to be balanced against the value of the remaining stored water.
 

Eagle Rock

Active Member
Well it can't really get below that level. Assuming that they left the outlet works open all of the way, the lake level would go up and down a little bit to push sufficient water through the outlet works to balance the inflow, just like any natural lake with an outlet. If they wanted to release more water through the dam below 3,370' elevation they would need to change something at the dam, like boring new bypass tunnels. (All of this assumes that there is not some secret design feature that would allow water releases below 3,370'.) There is very little stored water left at that level, somewhere between 700,000 af (water-data.com) and 1,700,000 af (1986 sedimentation survey), versus 24,322,000 full. The cost of bypassing the dam, the possibility of putting the dam back in commission, and so forth would need to be balanced against the value of the remaining stored water.
Well put, drewxmi, but I have a couple of tiny non-serious quibbles. First, there is a way that releases could stop entirely. If the lake elevation is at/near 3370', and at the same time evaporation exceeds inflow, then the lake surface could drop blow 3370' (I'm assuming "bank storage" is neutral at that point, since the lake elevation is not changing significantly). I took a quick look at the 24-month report from USBR re evaporation estimates, and it shows 55Kaf in July 2020 when the lake surface area was around 100,000 acres (per 1986 sediment study). At 3370', the lake surface area would be about 20,000 acres (also per the 1986 sediment study), or 1/5 as much. Assuming evaporation is proportional to surface area, that means peak-month evaporation at dead pool would be about 11 Kaf/month, which converts to about 180 cfs. So if inflow fell below 180 cfs, the lake could fall below 3370'! (I said that was a tiny quibble; I don't think real inflow has ever gotten anywhere near that low).
Second, you wrote that dead-pool storage is 1.7 MAF per the 1986 sediment study. I went back and checked. The study (https://www.usbr.gov/tsc/techreferences/reservoir/1986 Lake Powell Survey.pdf) shows the 1963 estimate of dead pool storage was 1.998 MAF (study page 7, Table 1, box 13g). That number was then adjusted twice in the 1986 study, first an upward increase of 33 KAF +/- 1 KAF to allow for water in the 1963 channel that wasn't counted in the origina; 1963 study, and then down by 142 KAF +/- 5 KAF to account for sedimentation through 1986 (see study p. 26, table 4, changes between columns 3 and 4 (upward adjustment) and value in column 7 (sedimentation). So the net dead pool number from the 1986 study is 1.879 MAF +/- 6 KAF, or 1,879,000 acre-feet. Again, just a quibble since if the lake ever gets to 3370', lots of upstream sediment will have been re-mobilized into the remaining reservoir, and the actual dead pool storage will be less than any of these numbers.
And that leads to a serious answer to a question posed above: why are there no bypass inlets below 3370? Because by the time the reservoir gets down to 3370', so much upstream sediment that's currently deposited between 3550' and 3700' will have been moved downstream that inlets below 3370' would be at risk of being blocked. The 1986 study shows 716 KAF of sediment already in place by then between 3380' and 3700', which implies about another 1000 KAF since then. All that sediment would be getting shifted downstream as the lake fell towards 3370.
 

KCYakker

Member
Very interesting stuff... thanks for sharing. Great photo comparisons, but the slide I keep coming back to is this graph on the 5th slide... that sediment profile is something to study. I had to do a little mental translating from meters to feet, but that amount of sediment up there is staggering... and you can really see how it's marched downstream with the falling surface elevation of Lake Powell....

View attachment 15442
Here's a question - has USBR done similar coring and surveys on the San Juan arm? As I understand it, sediment has moved downstream more quickly there, given the steeper gradient. Beyond that, I'm not familiar with what they may have discovered. The Colorado's the master stream, so I wouldn't be surprised if they'd concentrated most of their efforts on it.
 

Eagle Rock

Active Member
Here's a question - has USBR done similar coring and surveys on the San Juan arm? As I understand it, sediment has moved downstream more quickly there, given the steeper gradient.
Yes, in 1986. See p. 31 of the 1986 report (link in my post above) for a profile of the length of the San Juan, and p. 55 for two cross sections, one near the confluence with the Colorado and one much farther upstream. The USGS also did detailed bathymetric surveying in 2017, but I don't know where or if it is published in as readable a format as the 1986 USBR survey,
 

KCYakker

Member
Yes, in 1986. See p. 31 of the 1986 report (link in my post above) for a profile of the length of the San Juan, and p. 55 for two cross sections, one near the confluence with the Colorado and one much farther upstream. The USGS also did detailed bathymetric surveying in 2017, but I don't know where or if it is published in as readable a format as the 1986 USBR survey,
Cool, thank you!
 
Top