1. US Army Corps of Engineers BUILDING STRONG ® Boise River Feasibility Study Ellen Berggren, PMP Outreach Coordinator/ Project Manager Idaho Governor’s Roadless Commission Briefing June 10, 2015

2. BUILDING STRONG ® Study Authority Water Resources Development Act (WRDA) of 1999, Section 41 - Flood control authority WRDA 2007, Section 4038 - Added ecosystem restoration & water supply authority - Pre-agreement credit to Sponsor for $500K 2

3. BUILDING STRONG ® Non-Federal Sponsor Idaho Water Resources Board (IWRB)  Established by Idaho Constitution  Appointed by Governor  Develops and implements State Water Plans  Idaho Department of Water Resources provides project support and coordination  Pays 50 percent of feasibility study costs 3

4. BUILDING STRONG ® 4 Boise River Watershed Avg. Annual Runoff = 1,940 KAF System Storage = 950 KAF Arrowrock Dam Reclamation Completed in 1915 Storage = 286 KAF Anderson Ranch Dam Reclamation Completed in 1945 Storage = 418 KAF Lucky Peak Dam USACE Completed in 1954 Storage = 264 KAF

5. BUILDING STRONG ® Flood Risk Problems  Extensive floodplain development $10 billion of property at risk  Repeated minor flooding Protection to ~ 3% event from existing system  Largest population center in ID 40% of ID residents live in the Boise River Watershed 5

6. BUILDING STRONG ® Boise Rr Flood Hydrology 1943 – Flood in Eagle, ID Bankfull flow = 6,500 cfs (at Glenwood) Flood stage = 7,000 cfs (at Glenwood)

7. BUILDING STRONG ® 3% chance flood - 8,000 cfs 0.2% chance flood - 35,000 cfs 1% chance flood – 16,600 cfs 0.2% chance flood - Infrastructure at risk over $10 billion* 1% chance flood - Infrastructure at risk over $1 billion* Source: Ada and Canyon County Hazard Mitigation Plans, 2011 and 2013 7

8. BUILDING STRONG ® Water Supply Problems  Semi-Arid: 8-12” of precip. per year  High Population Growth: - 4.5% 1990 to 2000 - 3.3% between 2000-2010 - 4 th fastest growing state population  98% of existing DCMI water is from groundwater  Prohibitions on development of additional groundwater sources  Curtailment of junior water rights 8

9. BUILDING STRONG ® Water Supply Problems  Projected increase in water demand for the 50-year study period (estimated future needs additional 83,000 – 170,000 acre-feet)  Constrained storage system  Climate variability  Interconnected surface and ground water  Uncertain reliability of current and future water supply 9

10. BUILDING STRONG ® Study Objectives 1) Reduce flood damage and life safety risk 2) Increase operational flexibility and resiliency 3) Increase conveyance 4) Reduce the potential for severe erosion and/or sudden channel migration 5) Increase water supply to meet current and future water demands 10

11. BUILDING STRONG ® Alternatives MEASURES ALTERNATIVES ABCD No Action Arrowrock Dam Raise X XX Managed Aquifer Recharge X Upgrade Irrigation Headgates XXX Replace Push-Up Dams XX Upgrade Bridges X Controlled Flooding of Pits/Ponds X Temporary Conveyance of Water In Floodplain XXX Flow Split Structure X Enhanced Water Conservation Non-Structural Measures XX

12. BUILDING STRONG ® Arrowrock Dam Raise Multi-purpose Storage Flood risk management Water supply Potential Maximum Raise Dam raise up to 74 feet Up to 270,000 acre-feet increased storage About 6.5 miles of river inundated 12

13. BUILDING STRONG ® 13 Maximum Potential Raise Dam raise - 74 ft. Reservoir elevation increase - about 70 ft. Approx 270, 000 acre-feet additional storage. Approx. 6.5 additional river miles inundated

14. BUILDING STRONG ® Timeline January 2016Tentatively Selected Plan Milestone* February 2016Draft Feasibility Report / Environmental Impact Statement released for public review May 2016Agency Decision Milestone* December 2016Final Feasibility Report / Environmental Impact Statement March 2017Civil Works Review Board* June 2017Chief’s Report*

15. BUILDING STRONG ® Questions? Lucky Peak Project, 1954