2014 Water Quality Assessment and 303(d) List for
Using Fresh Water Data
Washington Submits 2014 Water Quality Assessment to EPA
View the 2014 Assessment:
Changing to a Water-based Segmentation System for this Assessment
New with this Assessment, we are updating how we map polluted rivers, lakes and streams. We are moving from a land-based segmentation system (township/range/section) to a water-based (National Hydrography Layer) system. The mapping change helps us be consistent with other agencies and states, and is strongly recommended by the Environmental Protection Agency. To help visually explain the differences between the land-based stream segments used in the 2012 Assessment with the water-based stream segments used in the 2015 Proposed Assessment,
we have created printable posters:
To assist with visually comparing the current 2012 Assessment listed segment with the proposed 2015 listed segment, we are providing a comparison tool on the
map viewer. For step-by-step instructions on how to use this review tool, go to:
Why are we changing the segmentation system for the Water Quality Assessment?
Washington State has adopted the National Hydrography Dataset (NHD) as the state standard hydrography. The NHD system provides an improved, higher resolution, hydrologic framework for our rivers and streams. In the 2008 Ecology made a commitment to EPA and others to convert the current segmentation system to NHD at the 1:24,000 scale when it was available. All other states in the nation have already make the conversion to a water-based system for listing segments.
What segment system has been used in past Assessments?
Going back to 1996, Ecology reported 303(d) listings by entire waterbodies. So, for example, the entire Yakima River was listed as polluted regardless of where the impairment occurred. This was considered too broad, and starting in 1998, Ecology changed to reporting the majority of fresh waterbody segments of rivers and streams as the portion of the waterbody lying within a given section of a township and range (T/R/S). This created much smaller segments based on where a waterbody flowed in and out of a section boundary. This political boundary system was used because Washington did not yet have an available hydrologic map layer that was consistent with EPA’s recommended stream reach-based system for segmentation of fresh waters.
What are the benefits of using the NHD water-based system?
NHD is used nationwide and is preferred by EPA as a “national” standard for mapping state water quality information.
Segments (Assessment Units) will be defined on a water-based system using stream reach boundaries, rather than on political or administrative boundaries that can appear arbitrary and that don’t reflect homogenous, or similar assessment unit.
This system will give compatibility with other state datasets by using the same NHD mapping layers.
Washington State has adopted the NHD as the state standard hydrography, so other Ecology programs and State and federal agencies are also using the NHD map layers.
How are Assessment units represented for large rivers like the Columbia and Snake that are currently gridded?
For this transition, large river assessment units have been defined for the Columbia and Snake rivers. This will give improved representation of the extent of water quality impairments, and provides a much better representation of flowing water than the grid cell system that was designed to apply to open waters such as large lakes and marine waters. Large river assessment unit listings are delineated by watersheds and their associated tributaries that flow to that part of the large river, creating a homogenous assessment unit for that water segment.
The large river assessment unit now includes the entire shoreline (similar to smaller streams), and can be more easily associated with dischargers when issuing permits. The current grid segment often does not connect to the shoreline and is difficult to associate with discharges to that part of the river.
Large river assessment units range in size from 1.1 to 19.4 miles in length.
How are the new NHD water-based segments different from the previous segment system?
The new NHD water-based segments are defined by establishing stream reach segments based on boundaries of upstream and downstream tributaries, also referred to as a “confluence to confluence” approach. The current segmentation system was based on administrative boundaries where a stream flows in or out of a township/range/section boundary.
Using the confluence-to-confluence approach creates a water-based segment that ensures that the water flowing within the segment is assumed to be homogenous, or similar in water quality attributes. When a stream flows into another stream, it potentially changes the water quality as the waters mix, and thus creates a new reach, or segment.
The change to an NHD water-based system now defines segments in a very different way from the current township/range/section approach. This will mean that for many streams, segments will now be longer, and in some cases (more rare) segments will be smaller.
Large river assessment units have been defined for the Columbia and Snake rivers. Listings are delineated by watersheds and their associated tributaries that flow to that part of the large river, creating a homogenous assessment unit for that water segment. The grid segment that is currently applied to large rivers was designed to apply to open (non-flowing) waters. Gridded segments represent a very small portion of the river and often do not connect to the shoreline, so it is difficult to determine discharges that may be flowing into that part of the river.
What does the change in segmentation system means for permitted dischargers?
The change to a water-based segment system could mean that some NPDES permitted discharges will now be discharging to a 303(d) listed waterbody segment when previously they were not and vice versa—some will no longer be on a 303(d) list. Being on the 303(d) list can result in additional conditions for a permitted discharger, depending on the pollutant at risk (for example, if the waterbody segment is 303(d)-listed for fecal coliform, a POTW may have additional limits so as to not add more of the pollutant to the impaired water).
How are the new segments defined?
Stream segments: The NHD water-based system established segments, also called assessment units, as a “Reach” and defines unique “Reach Code” numbers to that segment. A segment is defined as a reach for which a specific water quality standard applies. It is based on the full Reach Code whenever possible. The Reach Code may be subdivided into more than one assessment unit if the water quality standard changes in the middle of a segment. Reaches may be combined or split to create more than one assessment units if such a need arises. The segment lengths for streams range from 0.1 miles to 17.7 miles, with an average length of 1.6 miles.
Large River segments: The NHD system does not delineate large rivers, so it has been necessary to create large river assessment units that are based on the same water-based principles as that of NHD. Large river segments are delineated by the watershed and associated tributaries that flow to that reach of the river. The segment lengths on the Columbia and Snake rivers range from 1.1 – 19.4 miles. By comparison, in Oregon the Columbia River has assessment units that range from 35 to 197 miles, and in Idaho the Snake River has assessment units that range from 4 to 108 miles.
How can I compare the new segmentation system with the T/R/S segment system that is currently being used?
In the review map tool that we are providing for the user to review and comment on the proposed Assessment for fresh water data, a comparison tool has been created to compare the 2012 listing segment with the new water-based segment. In the excerpt from the map visual below, you will see “2013 Spatial Extent.” Click on the “compare” button and a comparison map showing the two segments will created.
This change may affect permitted facilities and we encourage regulated dischargers to use the mapping tool to understand what this change might mean to their specific facility.
Examples of the Change in Segments from T/R/S to NHD:
Little Soos Creek—2012 T/R/S segments
Little Soos Creek—2015 NHD segments
Examples of the Change in Segments for Large River Assessment Units from the current grid system to the large river assessment units:
Columbia River – 2012 gridded segments:
Columbia River – 2015 large river assessment units:
For questions about the water quality assessment process, please
Water Quality Program
Washington State Department of Ecology
P.O. Box 47600
Olympia, WA 98504-7600
Join Our ListServ to receive news and information about water quality
from the Department of Ecology.