CREDA Comments/Questions on 2008 High Flow Experimental Science Plan 

December 2007, with GCMRC responses

CREDA Comment/Question #1. There is inadequate information about 

specifically where the Sediment is currently stored (or will be in March 

2008 when the release would occur) to be certain that the release will 

not do more harm than good with respect to building eddy sand bars 

throughout the Marble and Grand Canyon reaches. In fact, as I discussed 

in some detail in my December memo, the available information indicates 

to me that the reaches downstream from Phantom Ranch (RM 87), and 

perhaps even downstream from the Little Colorado River (RM 61), are not 

sufficiently sediment-enriched to rebuild eddy sand bars in these 

reaches, and in fact, the proposed high flow would likely cause 

additional loss of sand bars in these reaches.

GCMRC Response: About 1/2 to 2/3 of the cumulative sand inputs that 

entered the system in the 18 months prior to the 2008 HFE were above 

Phantom Ranch gage. Another 300,000 to 500,000 tons were likely between 

that point and Diamond Creek. About 1/3 of the input sand had already 

passed the Diamond Creek gaging site. Although there was likely more 

sand in all reaches at the start of the 2008 test than existed prior to 

the 2004 test, it is possible that without enough sand below phantom 

ranch that sandbar erosion may have occurred and that the mass balance 

of sand below phantom could have become negative by the end of the 2008 

test flow. 

CREDA Comment/Question #2. Related to the above point, the Working 

Hypotheses section of Experimental Study 1.A (p18) includes the 

following statement: If reaches downstream from river mile 30 are sand 

enriched relative to their condition before the 2004 high flow, then 

sandbar building in these downstream reaches will be greater than was 

observed in these reaches during the 2004 high flow [emphasis added]. 

On the other hand, one of the conclusions in “The Importance of 

Tributary Floods” section (p4) is that the 2004 experiment resulted in 

an increase of total sandbar area and volume in the upper half of Marble 

Canyon, but further downstream, where sand was less abundant, a net 

transfer of sand out of eddies occurred that was similar to that 

observed during the 1996 experiment. The latter conclusion is 

consistent with the concern expressed in 1. Above.

GCMRC Response: We will request additional clarification from CREDA on 

April 8th regarding this question and will be happy to provide further 

response to this concern/question afterward. We admit that less sand in 

western Canyon could result in erosion of sandbars there. 

CREDA Comment/Question #3. The science plan should include more effort 

to quantify where the sand is stored along the reach prior to the high 

flow release. This should be done by adding the August 2007 and later 

data to the analysis presented in Slide 4 of Topping and Melis' WY2007 

Sand Supply Update presentation and Figure 1 from my December memo. If 

similar data are available farther downstream (e.g., at Diamond Creek), 

these data should also be added to the analysis. In addition, because 

the depositional response at a specific sand bar appears to be strongly 

related to the amount of sand storage in the pool(s) immediately 

upstream from that sand bar, detailed bathymetric surveys should be 

completed prior the high flow release to quantify local conditions in 

the vicinity of the bars at which detailed studies are to be performed. 
CREDA Comments/Questions on 2008 High Flow Experimental Science Plan 

December 2007, with GCMRC responses

(This may already be part of their plan, but I cannot tell for sure from 

the description.)

GCMRC Response: It was not feasible to add this information to the HFE 

science plan before the plan was completed in late December; mostly 

owing to the fact that the scientists who would have needed to provide 

the additional requested information were completely encumbered with 

preparation for the pre-HFE fieldwork and other preparations needed for 

during-HFE activities. This information will be presented to the TWG in 

2008 and will be included in results once they are completely finalized 

and reported to the GCD-AMP. 

CREDA Comment/Question #4. Continued detailed studies at the sand bars 

in Marble Canyon associated with Experimental Study 1B should provide 

valuable information for understanding eddy-sandbar dynamics. I 

appreciate the reasoning for limiting these studies to one specific area 

of the river, but I believe similar studies should be conducted farther 

downstream where the river is less (or perhaps NOT) sediment-enriched. 

In fact, one could argue that a relatively large body of data is already 

available at the proposed sites from the previous high flow releases, 

and more value could be gained by performing these studies farther 

downstream. If the studies were moved downstream to a suitable location 

in Grand Canyon, they would very likely provide data on the specific 

processes by which sand is depleted from sandbars during high flows in 

areas that are not sand-enriched, and this may actually show that the 

high flow releases are detrimental through a large portion of the Grand 

Canyon, even under sand-enriched conditions in the upstream part of the 

reach.

GCMRC Response: The primary objective of the research is to focus on how 

sandbars are formed under sand enriched conditions so that eventually 

"prescriptions" may be available to managers from modeling outcomes that 

optimize those responses. Hence, the 1.B studies were focused with 

limited available resources and staff on sites in a reach where we knew 

sand enrichment was relatively great (mid-marble canyon). We already 

have some data and understanding of how high flows under sand depleted 

conditions work to evacuate existing sandbars from eddies when the 

upstream sand supply is insufficient to deposit new sand in eddies. 

CREDA Comment/Question #5. The abstract for Experimental Study 1.A says 

that sediment concentrations and grain size will be measured at 7 fixed 

measurement sites, but the Methods section lists 5. Is this a typo, or 

are there actually 7 locations? If there are 7 locations, where are the 

other two?

GCMRC Response: The 1.A study measured those attributes at the following 

6 locations: lees ferry, 30-mile, 61-mile, 88-mile, 166-mile and 226-

mile gaging sites. Project 1 actually obtained these data at 7 

locations overall owing to the fact that study 1.B measured these 

attributes [and others] at 45-mile “Eminence Break” site. The study 1.A 

"Lagrangian" sampling trips measured these attributes throughout the 

entire CRE from two boats during the 2008 HFE.