Glen Canyon NRA 

• Update on 

Discussions of the 

Sloughs at -12-mile 

mark. 
 

 

 

 

 

 

 

 

 

Glen Canyon NRA Response to USBR -12-mile 

Slough Modification Proposal 

• Glen Canyon NRA (GLCA) has not yet decided on a pathway for modification of 

the sloughs at the -12-mile mark (3 miles downriver from GCD). 

• To make a decision, GLCA needs to better understand associated design, costs, 

equipment, restoration if feasible, and long-term maintenance obligations. 

NPS currently has no funds for modifying the sloughs. 

• NPS will further discuss the proposal and alternatives with Reclamation in 

November and December of 2023. 

• NPS will work with Reclamation to assemble an interdisciplinary “federal 

family” working group to start in January 2024 with an ecosystem perspective. 
Year 1889 - Sloughs at -12-mile Mark – Glen Canyon Reach 
Year 2011 - Sloughs at -12-mile Mark – Glen Canyon Reach 
 

 

 

 

 

 

-12-Mile Slough Considerations - GLCA 

• Significant natural feature (see 1889 photo) on the river 

• Wetlands – Riparian, shallow marsh, deep water with aquatic plants 

• Spring hydrology ? Underground upwelling? Bank feeder spring? 

• Two species of toads; one species of salamander (DNA sampling?) 

• Native fish species breeding (e.g. flannelmouth suckers) 

• Waterfowl feeding and resting (ducks, geese, shorebirds, others) 

• Christmas bird species count site, including foreign birds 

• Recreational hunting and fishing (Glen Canyon NRA multiple use) 

• Invertebrates, including dragonfly species. 
 

 

-12-Mile Slough Observations - GLCA 

• Upper slough remains full when river and lower slough are very low. 

• Non-native fish breeding not a problem in lower slough before 2022. 

• Only upper slough was managed for green sunfish problem b4 2022. 

• Water temperature rise in 2022 and 2023 brought SMB breeding. 

• Upper and lower sloughs warm earlier, longer & higher than river. 

• 2023 water temperature rise was earlier and longer than modeled. 

• Lots of aquatic vegetation in lower slough (6 acres in size). 

• 2 YOY size cohorts in lower slough in 2023 are larger than river YOY. 
Reclamation’s Proposed Slough Alternative 
 

 

 

 

Thoughts on Reclamation’s -12-mile Slough Proposal 

• Cut between upper and lower sloughs – more than hand shoveling 

• Significant elevation change from river bend to upper slough 

• Cut from river bend to upper slough longer and deeper 

• Armoring of inlet from river bend to upper slough? 

• Large amount of fill to narrow lower slough. Permit? Mitigation? 

• Initial or final restoration – native plants, select fauna, hydrology. 

• NEPA and other compliance – use prior documents? 

• Can we identify additional alternatives? (Low water? Pipes? Weirs?) 

• Work will require helicopter time, heavy equipment, and 

maintenance to be effective 

•
 

 

 

-12-Mile Slough Questions - GLCA 

• Shall we alter a significant natural feature? Habitat conversion? 

• Wetlands – Corps of Engineers full permit or nationwide permit? 

• Values to tribes - further consultation? 

• Spring hydrology – Retain for amphibians and invertebrates? 

• Reshape sloughs to retain or propagate native fish? 

• Mitigation for loss of waterfowl feeding and resting habitat? 

• Mitigation for loss of recreational hunting in Glen Canyon reach? 

• “Catch basin “ indicator location for invasive fish (“the canary”) – 

Will we push fish farther down the river if no “canary?” 
 

 

 

 

 

 

 

 

-12-mile Slough Closing Thoughts 

• For slough modification to have a chance to be effective, solutions 

at the source (Glen Canyon Dam) are vital. This means cooler water 

and eliminating passthrough of invasive fish. 

• 2016 documents aimed at Green Sunfish management mention a 

40% chance of slough modification being effective for bass. How do 

we raise the effectiveness of such modification? 

• What is our multi-agency commitment to a package of actions? 

• Consider feedback from tribes, Corps, state agencies, stakeholders 

• Plan for one or more rotenone treatments in July/August 2024. 

• Timelines – 8 months (2024) vs 1 year 8 months (2025) 

• What is the funding pathway for this significant effort? 
Questions? Comments?