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 the Degree of Hydrologic Alteration (DHA) concept proposed by Brian Richter and others (1996, see Technical notes) to evaluate the relative departure of stream flows from historic or normative conditions. For this particular indicator, the focus is on the degree of change in stream flashiness from historical conditions based on recent observations (1992-2009) of stream flow and modeled historic condition stream flow.

Because peak stream flow rises and falls more rapidly in urban areas and tends to have higher storm peak flows than forested areas, urban streams tend to have higher "flashiness" index scores. This "flashiness" is exacerbated by the generation of peak flows in urban streams during summer, which would not typically occur in forested streams. This increase in the "flashiness" index score represents the loss of water storage capability of soils and vegetation due to urbanization and the connection of paved surfaces and rooftops to streams via stormwater conveyance networks. To assess conditions throughout the county, "flashiness" was calculated each year for a set of 20 streams with long-term flow measurement records. The "flashiness" in each stream was compared to predictions from a hydrologic model that simulated stream flow under forested conditions. A mathematical comparison between the observations and the model predictions allow for an assessment of the Degree of Hydrologic Alteration at each stream flow measurement location.

Status: Flows from 20 stream sites in King County were measured and their "flashiness" calculated for the 2009 water year (October 2008-September 2009) and all other years for which data were available going back as far as 1992. Flows for seven of these streams were measured by the United States Geological Survey.

This indicator suggests that increased urbanization in King County has resulted in flashier stream flow response than previously occurred for most of the streams that have long-term stream flow monitoring data. In general, a high Degree of Hydrologic Alteration in stream flow flashiness has occurred in basins that are wholly or partially within the Urban Growth Area, which is consistent with the response of this indicator to urbanization. Stream basins that are wholly or predominantly outside of the Urban Growth Area tend to have a low Degree of Hydrologic Alteration in stream flow flashiness.

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 rainfall absorbing vegetation and soil are replaced by paved surfaces and rooftops connected to a conveyance system that routes rainfall runoff to streams. 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" lead 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.

The county's Stormwater Services group also implements stormwater retrofit projects designed to mitigate the effects of development on stream flow and water quality.

Priority new actions: In compliance with National Pollutant 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 flow, as well as water quality, is expected. This may translate into more monitoring at retention/detention ponds to make sure they are working as expected. More emphasis will also be placed on Low Impact Development (LID) techniques that minimize the amount of paved surfaces and rooftops that quickly direct water to streams and increase the opportunities for water to infiltrate into the ground. Examples of these LID techniques include green roofs, rain gardens, narrower streets and permeable pavement to name a few.


Degree of hydrological alteration in stream flow flashiness
1992 - 2009 Findings
Click to download the PDF version.

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 decreased in 2009.

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
2009 Findings
Click to download the PDF version.