A Flood Coming...


   There will be another flood next spring, scheduled for late March or early April. This time it will be on purpose. Officially labeled the 1995 Experimental Beach/Habitat-Building Flow, it will consist of about a week of 40,000- 45,000 cfs. Beach/Habitat-Building flows are proposed as part of the preferred alternative in the Glen Canyon Dam Draft EIS. There will be 4 days of 8,000 cfs before and after the flood in order to video the entire river. The release is timed to mimic predam flows while minimizing impacts to the nesting sites of the endangered Willow Flycatcher.

   It should be noted that this is designed as an experiment. Because we have never studied a flow of this type no one knows precisely what will be the result. Originally a flow of 52,000 cfs was proposed but the present low level of the lake precludes the use of the spillways and discharges are limited to 45,000 cfs. The hypothesis is that the high velocity water will lift the sand in the river bed and in the eddies and redeposit a portion of it on the shores up to the level of the river at flood. The energy of the river increases rapidly as the velocity of the water increases. Flow levels are critical and must be high enough to lift the sediment onto the beaches while carrying the least quantity downstream. Scientists at the United States Geological Survey feel the proposed flow will do just that. In addition, it is hoped that the high water will reexcavate and rejuvenate return channels, those deep channels on the back side on many sandbars that are key features to the rearing of native fishes.

   Below are two views on the upcoming flood.

 

Tom Moody

   I believe this experiment is critical to our learning how to manage the Colorado River through Grand Canyon. It should be conducted this spring. It is no secret that floods were an integral part of the natural processes of the pre-dam Colorado River. The natural system was one of erosion and deposition.

   From the beginning of the debate over operations of Glen Canyon Dam, GCRG has argued for two basic principles: that a positive sediment balance must be established, with more sediment entering the system than is carried out, and that some mechanism be found to periodically redeposit those sediments above river level. During the past 13 years the studies of GCES have allowed us to reduce the erosion dramatically. Reduction in fluctuations and the elimination of very large, clear floods has slowed erosion and allowed significant sediment to be stored in the river bottom. Now we need to redeposit those sediments and replenish beaches.

   There remain two questions: How big should the flood be and should it be next spring?

   No one really knows how high the water should go but 40-45,000 cfs seems reasonable to me. We are all concerned that irreparable damage be done to our sediment resource. Whether or not replenishing the beaches is successful, I doubt serious damage will result. I can't help but think back to the months upon months of similar or much higher flows between 1983 and 1986 and the fact that this flow will last only one week. Indeed, a better example would be the several days of high water we saw in early July 1980 with the filling of Lake Powell. Few if any of us noticed any loss of beaches that time. It would be nice to have the luxury of experimenting with several water flows, working our way up, but recording and analyzing the results is a very expensive proposition in both time and money. I feel confident that we will learn enough from this flow to determine its usefulness in future management.

   The experiment should be conducted this spring for two reasons. First it must be studied closely. That study will be far easier and less costly if it is done in conjunction with GCES studies that are scheduled to end next year. Secondly, now is the time precisely because the lake is low and the chance of an accidental spill remote. It is felt that Marble Canyon now has sufficient sediment to make success likely but an accidental spill would remove that. Because some sand will undoubtedly be carried downstream to Lake Mead, an uncontrolled flood shortly after this experiment might erode all the gains made.

   We should support this flood. There is much to gain and little danger. When the water subsides some beaches may quickly erode to their former size but it is very likely that we will rediscover the Colorado River is still the best tool for managing the downstream ecosystem.

 

Kenton Grua

   Why so high? It’s really pretty simple. A beach replacement flow should do just that. It should replace beaches where we need them, not take them out. We saw in 1983 after the 92,000 cfs flow that clear water can deposit sand. Sediment accumulates in eddies, but as the river level rises some eddies—the ones that are formed behind low cobble bars such as 110-Mile camp (RM 109.4 R) and low debris fans like upper National Canyon (RM 166.4 L) disappear and become fast moving downstream current. When this happens the sand scours and the beach is gone when the water drops back down to “interim flow” levels.

   The difference between a pre-dam silt-laden and a post-dam clear-water flood is that sand is only deposited during the rise and the peak of the flow and not to any extent as the flood subsides. This is due to the fact that the sediment comes from channel scour which increases exponentially with increase in flow and decreases as the flood subsides.

   We lost 110-Mile and National camps along with several other beaches with similar situations during the ’83 flood and it took several years of lower water (’86-’89) and then the moderately high water (high fluctuating test flows) to begin to replace them. They really came back after the LCR flood of 1993 which brought the river up to around 35,000 cfs below the confluence as well as contributed loads of sediment to the system during both the rise and fall of the flood.

   The theory is that you need at least 45,000 cfs to “really get the channel sand moving.” The problem is that somewhere around 40,000 certain camps become part of that channel sand and they won’t be there when the experimental flood subsides. The fact is that the “high fluctuating test flows” redeposited 110-Mile and National and several other camps that were lost in ’83 and that would indicate that maybe we don’t need to “really get the channel sand moving” to have a successful beach replacement flow.

   We need to determine what the flow is that will enhance the beaches in critical reaches. I propose that 110-Mile camp become the focus for determining the ideal beach replacement flow that provides deposition and not scour at this site. 35,000 cfs could be the upper limit.

   Maximum power plant capacity (32,000 cfs) might be adequate to do the job. 110-Mile camp should be monitored during the rise to determine the level most conducive to deposition and the flood not permitted to go any higher. This more conservative approach to the experimental beach replacement flow would also be more acceptable to other groups such as the power interests and trout fisherman. In addition, by using less of the channel sediment reserves we can have more frequent (annual or even biannual flows) to keep replacing the beaches which slowly erode away at the interim flow levels.