State of Our Waters - Streams
Snohomish County is home to over 5,700 miles of rivers and streams flowing down from the Cascade mountains and into Puget Sound. The State of Our Waters program helps us understand the health of this vast network of streams that provide both natural beauty and important functions including:
- Fish and wildlife habitat
- Flood prevention
- Groundwater recharge for drinking water
- Pollution prevention
Stream Health Report Cards
Monitoring Stream HealthSnohomish County looks at five key indicators to understand stream health. Three of the indicators, water quality, aquatic life, and habitat are summarized in the stream health report cards. The other two indicators, land cover and hydrology provide context to understand changes in health over time.
Water quality is a measure of the chemical and physical characteristics of the water that affect the streams health. Water quality includes measuring the chemical and physical characteristics of the water including the following eight parameters:
|- Temperature||- pH||- Nitrogen||- Turbidity|
|- Dissolved oxygen||- Fecal coliform||- Phosphorus||- Suspended sediment|
The results are combined to calculate the streams Water Quality Index (WQI). The annual index score is an average of the three lowest scoring months. The score can range from 0-100 with higher numbers indicating better water quality. The WQI was developed by the WA State Department of Ecology and is used throughout the state to summarize water quality data in an easy-to-understand format.
To assess the health of stream aquatic life, Snohomish County primarily examines the aquatic insects living on the stream bottom known as benthic macroinvertabrates. Most are insects that live part of their life in the water such as mayfly and dragonfly larvae but also includes things like snails and clams. In addition to being a vital part of the stream ecosystem, these organisms provide excellent indicators of health because they:
- Exhibit tolerance or intolerance to impacts in streams like pollution or habitat changes
- Are relatively sedentary and cannot easily move away from pollution
- Can be collected easily
Stream habitat is important for salmon, steelhead and other aquatic life. Habitat conditions for fish are assessed in the summer by looking at a representative stream length which varies based on width (100-800 meters). Measurements of the following features are taken continuously or at 11 points along the survey length and are combined to develop a habitat index score of 0 to 100:
- Large woody material – number of large wood pieces
- Pool habitat frequency – number of pools
- Pool habitat area – total pool area out of the total wet area
- Streambank armoring – percent of the streambank with riprap or similar armoring materials
- Streambed fine sediment – percent of streambed material that is sand or silt (<6 mm)
Land cover is the type of vegetation or surface on the land. Land cover influences stream health because runoff from rain flows across the landscape and into the stream. We characterize the types of land cover in the area that drains to a stream known as a the stream's watershed. Since land cover is closely related to land use, we also chose stream sites that represented the four main land uses in Snohomish County:
|- Forest||- Urban||- Rural||- Agriculture|
The land cover in the area nearest a stream, called a buffer, is especially important to understand stream health. Natural conditions in this area help to reduce pollution, provide shade to the stream, supply woody material to the stream habitat, and stabilize the stream banks. Evaluating changes in land use and the impacts on stream health provides data to help inform long term land use planning and decision making.
Hydrology data tells us how much water is flowing in our streams, largely from rain and snow melt, and how flow changes during the year. Flow measurements are taken at each site (frequency varies) and the data analyzed to determine information like:
- Low flows - particularly in the summer
- High flows - such as during a major rain storm
- Pulse counts – the number of high flow events
- Flashiness – how fast the stream flow increases and decreases during a storm
This information provides important context about what is happening in the stream as flow can impact water quality and aquatic life. For example, low summer flows are difficult for fish and higher winter flows can carry more pollution to the stream. .
Which streams do we monitor?Each year we randomly select 30-50 sites to sample. The sites are selected to represent the four major land use types in the area: urban, rural, forested, and agricultural areas. There are two types of sites:
- Trend sites - used to track changes over time and help to us understand if changes are due to large-scale natural causes, like drought or local, watershed-level changes. To do this, the same 20 or so sites are sampled every year.
- Status sites - used to understand the conditions of specific streams. Each year approximately 15 status sites are sampled and over time collectively help to understand the larger stream system.
Do you live near a stream? - we are here to help
As a streamside landowner you have a unique opportunity to help keep water clean, benefit wildlife habitat and improve flood and erosion control. Snohomish County has resources to help you protect your property and improve the health of our local waterways. For more information or to talk to our watershed steward visit our streamside living page.