Water Temperature Conditions in the Snohomish River Basin July 2021 Prepared for: Snohomish River Basin Salmon Recovery Technical Committee Prepared by: Josh Kubo, Andrew Miller, and Emily Davis. King County Water Land and Resources Division. Acknowledgements: Project Team: Emily Davis, Elissa Ostergaard, Kollin Higgins, Andrew Miller, and Josh Kubo Reviewers: Matt Baerwalde, Dave Beedle, Keith Binkley, Steve Britsch, Curtis DeGasperi, Aimee Fullerton, Kollin Higgins, Heather Kahn, Janne Kaje, Frank Leonetti, Kurt Nelson, Elissa Ostergaard, Colin Wahl. Recommended Citation: Kubo, J., A. Miller, and E. Davis. 2021. Water Temperature Conditions in the Snohomish River Basin. King County Department of Natural Resources and Parks, Water Land and Resources Division, Seattle, WA. July, 2021 Contents Executive Summary ................................................................................................................................... 3 Why is water temperature important for salmon recovery? ................................................................... 6 Drivers of Water Temperature ............................................................................................................... 10 Human Alterations to Aquatic Thermal Regimes ................................................................................... 16 Water Temperature Standards in the Snohomish River Basin ............................................................... 21 Water Temperature Conditions in the Snohomish River Basin .............................................................. 27 Overview of the Snohomish River Basin ............................................................................................. 27 Temperature Conditions - Snoqualmie River .......................................................................................... 28 Overview of the Snoqualmie River Watershed ................................................................................... 28 Above the Snoqualmie Falls ................................................................................................................ 29 Below the Snoqualmie Falls ................................................................................................................ 36 Temperature Conditions - Skykomish River ............................................................................................ 44 Overview of the Skykomish River Watershed ..................................................................................... 45 Mainstem Skykomish River ................................................................................................................. 46 Skykomish River Tributaries ................................................................................................................ 47 Temperature Conditions - Snohomish River and Estuary ....................................................................... 51 Overview of the Snohomish River and Estuary ................................................................................... 52 Mainstem Snohomish River and Estuary ............................................................................................ 53 Snohomish River Tributaries ............................................................................................................... 53 Strategies to Improve Water Temperature Conditions across the Snohomish River Basin ................... 57 Riparian Restoration ........................................................................................................................... 64 Increasing Channel and Floodplain Complexity .................................................................................. 64 Increasing Channel, Floodplain, and Tributary Connectivity .............................................................. 65 Current Monitoring ................................................................................................................................. 67 Future Research ...................................................................................................................................... 69 Efficiency Through Strategic Planning................................................................................................. 69 Improvements and Additions to Monitoring ...................................................................................... 70 Thermal Diversity ................................................................................................................................ 71 Linking Fish Use Data to Thermal Data ............................................................................................... 74 Investigating Watercourses of Concern .............................................................................................. 75 Literature Cited ....................................................................................................................................... 76 Appendix 1 – Water Temperature Monitoring in the Snohomish River Basin ........................................... 91 2 July, 2021 Executive Summary Aquatic temperature conditions affect all salmonid life-stages and influence their health, behavior, and survival. High water temperatures can be lethal to salmonids and warm temperatures can have a wide range of sub-lethal effects. The overarching importance of temperature conditions for salmonids in the Snohomish River basin emphasizes the need to fully address temperature impairments through conservation planning and strategy development. Human activities greatly impact aquatic temperature conditions and strategies which improve impairments and alleviate stressors will be critical for protecting and maintaining productive salmonid populations. Additionally, aggressive action may be needed in the near-term just to offset projected temperatures increases and impairments due to climate change. Several factors impact aquatic temperature conditions in the Snohomish River basin including land use development (clearing and impervious surfaces), removal of riparian buffers and broader forest cover, channel engineering and alterations, dams, water withdrawals, point-source discharges, and human- caused climate change. These factors can influence temperature conditions by increasing the susceptibility of watercourses to heating and by limiting the exchange and availability of cooler water sources. Additionally, these factors can alter the timing and magnitude of hydrologic and thermal inputs, which influence temperature conditions and thermal regimes throughout the basin. Various temperature monitoring efforts across the Snohomish River basin have provided useful insight into temperature conditions as well as potential strategies that may improve impaired watercourses. Continued temperature monitoring will be critical to further evaluate temperature conditions and trends moving forward. Observations across the Snohomish River basin indicate that there are widespread temperature impairments, primarily during summer months, with temperatures commonly exceeding Washington State water quality standards. Mainstem reaches of the Snoqualmie, Skykomish, and Snohomish rivers display frequent water quality temperature exceedances with impairments lasting up to several weeks or months. Additionally, several larger tributaries (e.g, Middle Fork Snoqualmie River, Pilchuck River, Raging River, Wallace River, and others) as well as smaller tributaries (e.g., Carpenter Creek, Cherry Creek, French Creek, Patterson Creek, Woods Creek, and others) also display consistent temperature exceedances and in some cases reach lethal levels. The Snohomish River Estuary also displays impaired conditions, partly due to natural condition (e.g., warming on mudflats), with summer temperatures at times being the highest across the entire Snohomish River basin. Observed temperature impairments across mainstem and tributary areas have largely been attributed to factors described above. Despite the wide-spread occurrence and persistence of warm summer temperature conditions across the Snohomish River basin, several mainstem reaches, tributaries, and floodplain areas display considerably cooler and less impaired temperature conditions. Areas of relatively cooler temperatures may provide thermal refuge for salmonids and have the potential to lower temperature among adjacent watercourses and mainstem areas. Mainstem reaches with temperature moderation and cooling include the middle and lower reaches of the North and South Fork Snoqualmie River, upper mainstem Snoqualmie River from the South Fork Snoqualmie to Snoqualmie Falls, lower Snoqualmie River from Patterson Creek to downstream of the Tolt River, North Fork Skykomish River, Skykomish River around 3 July, 2021 the confluence with the Sultan River, as well as select middle reaches in the Pilchuck River. Cooling among these mainstem reaches is likely associated with decreased solar exposure due to topographic and riparian shading, cold groundwater inputs, tributary inputs, as well as increased exchange among surface and sub-surface waters (i.e., hyporheic exchange). Tributaries with relatively cooler temperature conditions, compared to mainstem reaches, include the Tolt River and Sultan River, as well as several smaller tributaries (e.g., Boxley Creek, Griffin Creek, Tokul Creek, Worthy Creek, Quilceda Creek, and others). Cooler temperatures among these tributaries are likely due to less-degraded riparian and forest