THE IMPACT OF DAMS ON FLOODS AND NITROGEN FLUX IN THE LAMPREY RIVER WATERSHED, NH

xx Chapter 1: Introduction 1 1.1 Background Information 1.1.1 Dam Removal Tradeoffs . 1 1.1.2 Tools Used to Quantify Dam Removal Tradeoffs . 1 1.1.3 Effects of Dams on Flood Peak Attenuation . 2 1.1. 4 Modeling Hydrologic Alteration by Dams . 3 1.1. 5 River Network Nitrogen Retention . 3 1.1. 6 Effects of Dams on River Network N Retention . 5 1.1. 7 Modeling N Retention . 6 1.1. 8 Effect of Climate Change on Flooding and N Fluxes . 7 1.1. 9 Effect of Land Use Change on Flooding and N Fluxes . 8 1.2 Lamprey River Watershed Site Description 1.2.1 Watershed Overview . 8 1.2.2. Impact on Great Bay . 9 1.2.3 Watershed Management Plan and Instream Flow Policy . 11 1.2.4 Dams . 14 1.2.5 Historic Land Cover and Population . 16 1.2.6 Historic Climate and Flooding . 18 1.2. 7 Historic Nitrogen Flux . 19 1.2. 8 Historic Fish Runs . 21 1.3 Research Objectives . 22 Chapter 2: Field Methods 24 2.1 Pawtuckaway Lake Site Description 2.1.1 Watershed and Reservoir Overview . 24 2.1.2 Dam Management . 29 2.2 Field Methods Overview . 30 2.3 Tributary N Inputs and Dam Outlet N Outputs 2.3.1 Sampling Locations . 31 2.3.2 Continuous Stage Measurements . 32 2.3.3 Field Discharge Measurements and Uncertainty . 36 2.3.4 Stage-Discharge Rating Curves and Uncertainty . 39 2.3.5 Continuous Discharge Records and Uncertainty . 39 2.3.6 Discharge Record Gap Filling and Uncertainty . 41 2.3. 7 Grab Sample Measurements of N Concentration and Uncertainty . 47 2.3. 8 Estimates of N Flux at Monitoring Stations . 48 2.3. 9 Tributary Discharge and N Inputs at Unmonitored Tributaries . 49 iv 2.4 Nearshore Hydrology and N Inputs from SWAT Modeling 2.4.1 SWAT Model Overview . 50 2.4.2 SWAT Input Data . 50 2.4.3 SWAT Model Calibration . 52 2.5 Atmospheric N Inputs and Uncertainty . 58 2.6 N Mass Balance, Retention, and Uncertainty . 59 Chapter 3: Field Results and Discussion 61 3.1 Pawtuckaway Lake Water Balance . 61 3.2 Monitored Tributary and Dam Outlet N Solute Concentrations . 70 3.3 Tributary and Dam Outlet N Flux . 81 3.4 N Loading from Atmospheric Deposition and the Near Shore Loading from SWAT 91 3.5 Reservoir N Retention Results and Discussion . 93 3.6 N Flux from the Pawtuckaway Lake Watershed . 95 Chapter 4: FrAMES Modeling Methods 98 4.1 FrAMES Model Background . 98 4.2 Model Application Overview . 99 4.3 Model Input Data 4.3.1 River Network . 99 4.3.2 Dam Management . 100 4.3.3 Contemporary Climate and Land Use . 104 4.3.4 Future Climate and Land Use . 106 4.4 Model Calibration 4.4.1 Hydrologic Calibration Methods . 114 4.4.2 Hydrologic Calibration Results . 119 4.4.3 DIN Loading Calibration . 121 4.4.4 Modeled DIN Reservoir and River Retention . 124 4.4.5 DIN Concentration Variation with Discharge . 125 4.4.6 Modeled DIN Flux . 128 4.4. 7 Estimation of Hydrologic Parameter Uncertainty . 128 4.5 Model Scenarios . 129 4.6 Comparison Metrics . 131 Chapter 5: FrAMES Modeling Results 133 5.1 Flood Magnitudes 5.1.1 Dam Scenarios Under Contemporary Climate & Land Use . 133 5.1.2 Dam Scenarios Under Late-Century Climate and/or Land Use . 135 5.2 Winter DIN Load, Flux, and Retention 5.2.1 Dam Scenarios Under Contemporary Climate & Land Use . 137 5.2.2 Dam Scenarios Under Late-Century Climate and/or Land Use . 140 5.3 Summer DIN Load, Flux, and Retention 5.3.1 Dam Scenarios Under Contemporary Climate & Land Use . 143 5.3.2 Dam Scenarios Under Late-Century Climate and/or Land Use . 146