Lake Sammamish Water Quality Response to Land Use Change December 2014 Alternate Formats Available Cover photo: Top right corner: 2013 Pictometry. The dataset consists of tiled orthogonal imagery produced from nadir images captured by Pictometry International. This ortho-mosaic is a visualization product and is not intended or valid for authoritative or definitive use. Bottom left corner: 1936 Aerial Photos for western King County panchromatic imagery. The source is the original hardboard-mounted aerial prints. The original 0.5 foot pixel resolution was resampled to 1 foot pixel. The purpose of the imagery is historical comparison and archiving original documents. Lake Sammamish Water Quality Response to Land Use Change Submitted by: Eugene B. Welch Emeritus Professor, University of Washington Consulting Limnologist Debra Bouchard King County Science and Technical Support Section Water and Land Resources Division Department of Natural Resources and Parks Seattle, WA 98104 Lake Sammamish Water Quality Response to Land Use Change Acknowledgements Water quality sampling and analysis was conducted by the King County Environmental Laboratory. We would like to thank Jeff Droker and Jean Power for their help sifting through boxes of historical data records and for describing detailed sampling protocols. We are also grateful to Katherine Bourbonais for her assistance coordinating our data needs and making sure our questions were answered. We would like to thank staff in the King County Science and Technical Support Section for their help. Rachael Gravon and Andrew Miller provided help with data analysis. Sally Abella, Curtis DeGasperi, Jim Simmonds, and Kate O’Laughlin provided significant contributions with their technical review. A special thank-you goes to Jonathan Frodge with Save Lake Sammamish for his technical review. Warren Perkins authored the Lake Sammamish Total Phosphorus Model, frequently cited in this report as King County 1995. Project funding was through the Major Lakes Monitoring Program, supported by the Science and Technical Support Section of King County Department of Natural Resources and Parks’ Water and Land Resources Division. E.B. Welch and D. Bouchard i December 2014 Lake Sammamish Water Quality Response to Land Use Change Citation King County. 2014. Lake Sammamish Water Quality Response to Land Use Change. Prepared by Dr. Eugene B. Welch, Emeritus Professor, University of Washington, and Consulting Limnologist, and Debra Bouchard, Science and Technical Support Section, King County Water and Land Resources Division, Seattle, WA. E.B. Welch and D. Bouchard ii December 2014 Lake Sammamish Water Quality Response to Land Use Change Table of Contents EXECUTIVE SUMMARY ................................................................................................................................... viii 1.0 INTRODUCTION ...................................................................................................................................... 1 1.1 Study Purpose ..................................................................................................................................... 1 1.2 Lake Sammamish Characteristics ................................................................................................ 2 1.3 Background .......................................................................................................................................... 4 1.3.1 Past Studies .................................................................................................................................... 5 1.3.2 Watershed Changes ..................................................................................................................... 8 1.3.3 Increased and Ongoing Development ................................................................................10 1.3.4 Lake Sammamish Initiative and Management Plan ......................................................17 2.0 METHODS ................................................................................................................................................18 2.1 Lake Sampling ...................................................................................................................................18 2.2 Stream Sampling ..............................................................................................................................21 2.3 Laboratory Analysis ........................................................................................................................23 2.4 Statistical Analysis ...........................................................................................................................23 2.5 Water Quality Parameters ............................................................................................................24 2.5.1 Temperature ................................................................................................................................24 2.5.2 Dissolved Oxygen .......................................................................................................................25 2.5.3 Phosphorus ...................................................................................................................................26 2.5.4 Sediment Phosphorus Recycling and DO Demand ........................................................27 2.5.5 Nitrogen .........................................................................................................................................28 2.5.6 Conductivity .................................................................................................................................28 2.5.7 Alkalinity .......................................................................................................................................29 2.5.8 pH .....................................................................................................................................................29 2.5.9 Transparency ...............................................................................................................................30 2.5.10 Phytoplankton and Chlorophyll a ........................................................................................30 2.5.11 Total Suspended Solids ............................................................................................................30 3.0 RESULTS ...................................................................................................................................................32 3.1 Water Inflows ....................................................................................................................................32 3.1.1 Precipitation.................................................................................................................................32 3.1.2 Issaquah Creek Flows ...............................................................................................................34 E.B. Welch and D. Bouchard iii December 2014 Lake Sammamish Water Quality Response to Land Use Change 3.1.3 Tributary Water Quality ..........................................................................................................35 3.1.4 Tributary Phosphorus Loading ............................................................................................38 3.1.5 Residence Time and TP Load ................................................................................................39 3.2 Temperature ......................................................................................................................................39 3.3 Alkalinity, pH, and Conductivity .................................................................................................42 3.4 Trophic State Indicators ................................................................................................................45 3.4.1 Phosphorus ...................................................................................................................................45 3.4.2 Nitrogen .........................................................................................................................................51 3.4.3 Transparency and Chlorophyll a (chl a) ...........................................................................54 3.4.4 Areal Hypolimnetic Oxygen Demand .................................................................................58 4.0 DISCUSSION ............................................................................................................................................62 4.1 Ongoing Recovery from Wastewater Diversion ..................................................................62 4.2 Phosphorus and Development ....................................................................................................62 4.3 Trophic State ......................................................................................................................................66 4.3.1 TP and Chl a ..................................................................................................................................66 4.3.2 DO Demand ...................................................................................................................................68 5.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................................70 6.0 REFERENCES ..........................................................................................................................................73