Washington Criteria Rule Horsley Witten Group, Inc.

Calculation Summary June 30, 2015

Washington Criteria Rule

Percentage of Washington Waters that Flow into Oregon Waters via the Columbia River

Calculation Summary

Purpose: Quantify the percent of Washington’s fresh waters that could impact the water quality of waters under 

Oregon’s jurisdiction. Waters that flow into Oregon only do so via the Columbia River, which forms the majority 

of the border between Washington and Oregon. Since the two states share jurisdiction of the Columbia River, the 

water quality of tributaries to and portions of the Columbia River under Washington’s jurisdiction can influence

the water quality of waters under Oregon’s jurisdiction. 

Numeric Results:

 Stream miles that flow from Washington via the Columbia River into Oregon: 145,424 miles

 Total stream miles in the State of Washington: 241,064 miles

 Percent of Washington waters that flow into Oregon via the Columbia River: 60.3%

Data Sources:

 Watershed boundaries in Washington were obtained from USGS Hydrologic Unit Maps, 

(https://water.usgs.gov/GIS/huc.html);

 Streams in Washington were downloaded from USGS National Hydrography Dataset 

(http://nhd.usgs.gov/wbd.html);

 Washington State boundary was downloaded from the Washington Department of Ecology 

(http://www.ecy.wa.gov/services/gis/data/data.htm). This site provided the best most representative layer 

allowing for separation of land and water features in the state; and

 Major streams in Washington (http://www.ecy.wa.gov/services/gis/data/data.htm). The purpose for this 

layer was as a visual representation of streams and rivers for the map, not an all inclusive layer of 

streams; the USGS Hydrography Dataset was obtained for that purpose. 

General Steps:

1. Identify the watersheds within the State of Washington, at the HUC 4 level. The HUC 4 level was used 

because it is an appropriate size to identify and delineate watersheds in a state the size of Washington. 

2. Determine which major waterbodies potentially flow into Oregon, such as the Columbia River, the Snake 

River and any major tributaries. Since the Columbia River forms most of the boundary between the states 

of Washington and Oregon, streams, rivers and tributaries that flow into the Columbia River will

ultimately flow into Oregon;

3. Identify streams that flow into these major waterbodies;

4. Limit stream lengths to the State of Washington (i.e., exclude sections of streams outside state 

boundaries). This was done by clipping the stream datalayer (NHD24Kst_l_wa) with the Washington 

state boundary shapefile;

5. Calculate the total length of all streams within the state of Washington, using the clipped stream layer. 

This was done in ArcGIS, in the attributes table, using the “calculate geometry” option. The streams 

located in the HUC4 watersheds that do not drain into the Columbia River were selected and the total 

stream lengths were calculated. The HUC4 watersheds that drain into the Columbia River, including the 

Columbia River, were selected and the total length of the streams was calculated. 

6. Calculate a length ratio of the streams, identified in step 5, that flow into the Columbia River to total

length of the streams in Washington ( both inclusive of the Columbia River). Due to limitations of GIS, 

some segments of some streams (<10%) are missing from the calculations. 
Washington Criteria Rule Horsley Witten Group, Inc.

Calculation Summary June 30, 2015

Map Illustration of the Data:

Main map of Washington includes: 

 Major streams and rivers (purple lines);

 Watersheds that contain rivers and streams that flow into Oregon via the Columbia River (orange

watersheds);

 Waters that do not flow into the Columbia River (yellow watersheds); and

 Columbia River (teal line).

Inset map includes:

 Detailed view of the streams and rivers in Washington (dark blue lines); and

 Columbia River (teal line).