Effects of Island Park Reservoir on sediment and phosphorus transport in the upper Henrys Fork watershed Rob Van Kirk Senior Scientist Henry’s Fork Foundation Ashton, Idaho Ken Lindke Statistical Consultant

U.S. Bureau of Reclamation –Clyde Lay, Megan Hecket, Mike Beus HFF Staff, Volunteers, and Interns –Taylor Burdge, Jeff Clemente, Matt Cahoon, Anne Marie Emery, Trevor Gordon, Sheryl Hill, Chau Ho, Sara Reese, Thacia Schmidt, Blair Tynes, Chris Wynn, Funders –Cross Charitable Foundation –CHC Foundation –Ishiyama Foundation –Spruance II Foundation –Individual donors to Campaign for Wild Trout Acknowledgments

Outline 1.Background 2.Objectives 3.Study area 4.Methods 5.Results a)Hydrology b)Suspended sediment c)Phosphorus 6.Conclusions

Background 20 miles of Henrys Fork downstream of IP Dam supports world-famous wild rainbow trout fishery Flat water of Harriman State Park is most popular Henrys Fork fishery worth $30-60 million annually Drawdown of Island Park Reservoir to minimum pool in 1992 delivered 50, ,000 tons of sediment to river Drawdown in 2013 was forecast at 10% of capacity

Objectives 1.Estimate suspended sediment (SS) load to river in late summer/early fall Estimate total phosphorus (TP) load to river 3.Estimate SS and TP loads into/out of IP Reservoir in Investigate relationships among SS, TP, flow, and reservoir volume 5.Infer physical/chemical/biological processes governing sediment and phosphorus fluxes 6.Relate TP to instream productivity (to be continued in 2016)

Henrys Fork Watershed

Study Area: Upper Henrys Fork

Methods 2013 –Sample SSC, TP, and turbidity at Flatrock and IP Dam 4 days per week, mid-Aug to mid-Oct –Sample multiple locations across channel (result: <1% of variability due to location in channel) 2014 (and ongoing…) –Add Sheridan Cr. site to sample input of tributaries to west end of reservoir –Ungaged trib inflow = Total reservoir inflow – HF inflow –Sample SSC, TP, OP, and turbidity 2 days/wk in center of channel (depth-integrated) –Establish permanent multiparameter sonde sites ARIMAX time-series models used for inference of concentrations between sampling dates

Hydrology—Mean hydrographs

Concentration vs. Flow

Suspended Sediment Loads: 2014

SSC versus Reservoir Volume

Comparison of 1992 and 2013

Total Phosphorus Loads: 2014

Total Phosphorus vs. SSC: 2014 TP positively correlated with sediment upstream of reservoir No correlation between TP and sediment below reservoir Note same axis scales on all three graphs Horizontal line shows 0.1 mg/L for reference

Fraction of TP as OP

Load summary SSTPSSTPSSTP Inflow NA ≈ 152≈ 2.7 Outflow 50, ,000 NA658 [640,735] 11.3 [11.2,11.6] ≈ 350≈ 6.4 Loads (tons) over the period Aug 12 to Oct 17. Where available, 95% CI is given in brackets. SS loads several orders of magnitude smaller than 1992 SS and TP loads ≈ 45% lower in 2014 than 2013 Irrigation demand was very high in 2013 Net export of both constituents in late summer/early fall

What happens to all of the P? Stream bottom in March. Same location in September. Seasonal growth of macrophytes in low-gradient reaches.

Macrophyte cover in Harriman Reach Differences between years due to weather differences.

Summer 2014 Dissolved Oxygen Highest productivity is between IP Dam and Pinehaven, in Harriman reach.

Conclusions Sediment and associated phosphorus is stored in reservoir during runoff SS and TP concentrations below reservoir independent of discharge or volume except when volume is < 60k ac-ft (45% of capacity) TP concentrations in river highest during late summer, when high fraction is OP SS and TP loads below reservoir higher during dry years, when irrigation delivery is high 10+ yrs of data will yield P and SS budgets… High P export drives high system productivity based on macrophytes, at least for now…

Questions?