Water Quantity

About this indicator: King County's Water Quantity Index is derived from two main groupings of freshwater results describing the conditions of rivers and streams and groundwater. Lakes and wetlands do not factor into the index at this time. Our weighting system applies 80 percent to rivers and streams and 20 percent to groundwater condition results toward the overall water quantity rating. The weighting of groundwater quantity would be larger if data for groundwater well water levels for other areas besides Vashon-Maury Islands was collected on a regular basis. Although, there is no indicator for the marine environment, an indicator may be added next year with respect to sea level.

Status: Overall below standard with some areas of lesser concerns.

Influencing factors: Extensive development can substantially alter stream flow patterns and how they respond to rainfall. Changes in land use and/or vegetation, increases in groundwater withdrawals and climatic changes can adversely affect the quantity of groundwater.

What you can do: Practice conservation with respect to groundwater usage, low-water use gardening, adhere to regulations related to groundwater pumping, and support efforts to practice habitat restoration and best management practices to mitigate runoff resulting in flash flooding and channel erosion.

More information about King County's Water Quantity Index is available by continuing below for these measures:

• Normative Flows on Streams & Rivers
• Groundwater Water Levels on Vashon-Maury Islands

Normative Flows on Streams & Rivers

About this indicator: This indicator uses a stream "flashiness" index — based on the reciprocal of the fraction of days during the year that the flow rises above the annual mean daily flow. Because peak stream flow rises and falls more quickly in urban areas than forested areas, urban streams tend to have a smaller fraction of days during the year when the flow is above the annual mean daily flow, and a higher "flashiness" index score. This increase in the "flashiness" index score represents the loss of water storage capability of soils and vegetation due to urbanization. To assess conditions throughout the county, the median stream "flashiness" is calculated each year across all streams where flow is measured. The median stream "flashiness" score represents the degree of water storage ability where half of the streams are flashier and half are less flashy.

Status: Flows from 20 stream sites in King County were measured and their "flashiness" calculated during the 2008 water year (October 2007 — September 2008). Flows for five of these streams were measured by the United States Geological Survey. The median of the "flashiness" index scores across all streams measured in King County has increased between 1945 and 2008 and was lower in 2008 than in 2007.

These data suggest that increased urbanization in King County has resulted in faster surface runoff and peak stream flow rise and fall than previously occurred for some of the streams

Influencing factors: Extensive development can substantially alter stream flow patterns and how they respond to rainfall. In urban areas, surface runoff occurs more quickly than in forested areas because less rainfall is absorbed by the vegetation and soil. Faster runoff in urban areas results in higher peak stream flows rising and falling more rapidly than under forested conditions. Increased peak flows and "flashiness" leads to the most obvious effects from a human perspective — flash flooding and channel erosion. From a biological perspective, streams with greater "flashiness" are disturbed more often. Organisms that survive in these conditions are those that have adapted to more frequent and severe disturbances.

Existing DNRP response: King County has a range of regulatory, educational, and on-the-ground programs to reduce the impacts of development on streams and reduce the amount of "flashiness." The County's Drainage Design Manual directs drainage requirements for all new development.

Priority new actions: In compliance with National Pollutant Discharge Elimination System permit requirements from the state (as part of the federal Clean Water Act), a closer linkage between the effectiveness of stormwater controls and water quality and flows is expected. This may translate into more monitoring at retention / detention ponds to make sure they are working as expected.

Groundwater Water Levels on Vashon-Maury Islands

About this indicator: King County has been tracking groundwater quantity on Vashon-Maury Island since 2001. Water levels are tracked frequently in both volunteer and dedicated monitoring wells. King County's goal is to ensure sustainable water quantity through appropriate zoning regulations and high water quality through effective land-use and on-site septic regulations.

Status: Groundwater levels are generally steady, with some areas increasing and some decreasing.

Influencing factors: Changes in land use and/or vegetation, increases in groundwater withdrawals and climatic changes can adversely affect the quantity of groundwater. Changes in 2007 water levels are also thought to have been caused by reduced precipitation/recharge to island aquifers.

Existing DNRP response: King County plans to continue monitoring Vashon's wells and springs annually for water levels measurements.

Priority new actions: Additional locations have been sought to take water level measurements and increase our understanding of island aquifers. King County intends to produce Vashon-Maury Island-wide water table, contour maps with seasonal variability that will be reported every year.

Groundwater Water Levels on Vashon-Maury Islands
2008 Findings
Click to download the PDF version.