State of Our Waters - Streams

 

  Aquatic Life

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

The stream is sampled at eight different locations where all macroinvertebrates in a one square foot area are collected and then later identified. The information is used to assign the stream a health index score ranging from 1 to 100 with higher numbers indicating healthier streams. The index, known as the Benthic Index of Biotic Integrity (BIBI), is widely used to measure stream health and has specifically been adapted for the Puget Sound Region

   

Habitat

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: 

 

Land Cover

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

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. .