Water Quality Improvement Project
Lower Yakima River Area:
Suspended Sediment

Water quality issues

For several decades high suspended solids, turbidity, DDT and other pesticides, high temperatures, and other kinds of pollution have been documented in the lower Yakima River. By the mid-1990s water quality evaluations by the US Geological Survey (USGS) indicated that some improvements had been made, but beneficial uses were still impaired by sediment and sediment-borne pollutants, like DDT, from irrigation returns (Rinella et al., 1992b, 1993). Consequently, several reaches of the lower Yakima River and several of its tributaries did not meet numerous state water quality criteria and federal guidelines (Ecology, 1994a, 1995). As a result, Ecology placed these water bodies on Washington State's 303(d) list.

Fishing Platforms at the SVID Diversion, lower Yakima River.  Photo courtesy of Jane Creech, WA Department of Ecology.

Why this matters

Turbidity reduces light penetration and can interfere with natural productivity in the river. Turbidity also makes it more difficult for fish to move, avoid predators, and find food.

Suspended solids impair fish and aquatic insect respiration. Particles can also settle and clog spawning gravel or suffocate fish eggs.


What has been done

In the late 1990s, Ecology determined that turbidity and DDT represented key water quality impairments on the 303(d) list in the lower Yakima River basin. In response, Ecology conducted a TMDL study to evaluate controls of suspended sediment, the primary cause of the turbidity criteria violations, and a major source of DDT transport in the lower basin during the irrigation season. The TMDL was coordinated with the Yakama Nation (YN), since the Yakama Indian Reservation covers over forty percent of basin, but is outside of the state's jurisdiction.

In 1997, Ecology published a total maximum daily load (TMDL) about the lower Yakima River: Lower Yakima River Suspended Sediment TMDL. EPA approved the TMDL for the protection of chronic aquatic life criteria in 1998. The report details the amount and sources of several pollutants in the lower Yakima, and set targets for amounts of sediment and pesticides in the river that must be met during the irrigation season. Two different targets were set to protect aquatic species:

  • Turbidity, or cloudiness, of water.
  • Pesticides (DDT).

The TMDL noted that the relationship between turbidity, suspended sediment, and DDT would likely change significantly after most of the suspended sediment was removed from the river. Therefore, the TMDL intended to identify specific human health load allocations for DDT starting in 2007.

During the review and comment period for the 2004 water quality assessment, EPA re-evaluated this TMDL and made the following statement in their assessment comment letter to Ecology:

As explained in the 1997 Lower Yakima TMDL, the pesticide targets developed in this TMDL are derived from the chronic aquatic life criteria, and not the more stringent human health criteria. EPA's approval letter (11/25/98) specifically approves the Lower Yakima TMDL for chronic aquatic life (not human health). Because the TMDL was not written to meet the more stringent human health criterion, those waters for which data exists showing that human health standards are not being met need to remain in Category 5 until a TMDL is developed to meet a human health target. This comment applies to DDT, DDE and DDE listings on the Yakima River, Snipes Creek, Spring Creek, Granger Creek, Sulphur Creek, Side Hollow Creek [sic], and Moxee Drain.

Note: In the above excerpt, "Side Hollow Creek" should actually read as "Wide Hollow Creek".

Based on this determination, in 2005 moved all the DDT listings associated with this TMDL from Water Quality Assessment category 4A (has a TMDL) to category 5 (needs a TMDL).

Status of the project

Additional data were obtained from the USGS, US Bureau of Reclamation (USBR), the YN, and the North and South Yakima Conservation Districts. Monitoring and evaluation focused on drains and tributaries in the most heavily irrigated areas of the lower Yakima. Data evaluation, jurisdictional issues, and the TMDL strategy were discussed with, and reviewed by technical staff from the Ecology Central Regional Office (CRO), Region 10 of EPA, and the YN Environmental Protection Program.

Since completing the TMDL, many entities have successfully worked to reduce the amount of pollution returning to the Yakima River with irrigation return flow. The Roza-Sunnyside Board of Joint Control implemented a water quality policy, a water quality monitoring program, and used State Revolving Loan funds to help finance on-farm irrigation upgrades that significantly reduced suspended sediment and DDT loading to the Yakima River. The North Yakima Conservation District assisted with the conversion of over 8,000 acres of hop fields to drip irrigation seasons. All of this, along with much work and investment by Yakima Valley farmers, resulted in significant improvements in water quality for the Yakima River.

In 2003, Ecology conducted effectiveness monitoring in the Yakima River and the major tributaries with targets set by the TMDL. Turbidity and suspended sediment levels are much improved in the lower Yakima River watershed. Three out of four irrigation return drains met targets set by the TMDL. Suspended sediment and turbidity in the main stem Yakima river was considerably improved.

In 2006, Ecology began a large scale project titled "the Yakima Toxics Project" which will address the DDT removed from the sediment TMDL, as well as other toxic compounds.

Ecology believes the control of suspended sediment generation and transport during the irrigation season has resulted in far-reaching water quality and fish habitat improvements in the Yakima Basin. Numerous pesticides in Yakima River water and bottom sediments still pose a threat to both animals and people. The problem pesticides include DDT, which enters the Yakima River attached to suspended soil particles. The Yakima Toxics Project will set refined loading targets for turbidity from irrigation return flow and will set pollutant reduction targets from other sources such as stormwater.


Since a high percentage of the pollution in the lower Yakima still comes from erosion of soil from farms, growers that have not made changes in irrigation practices and irrigation water management need to do so now. Sediment from farmland, including pesticides that adhere to the sediment particles, must be reduced more in major drains and tributaries to meet targets outlined in the Ecology report. Other sources will be assigned load reduction targets as part of the Yakima Toxics Project.

What will be done

Based on the findings and recommendations in the Yakima Toxics Project technical study, Ecology is developing a new water quality improvement project that includes a new evaluation of current levels of DDT in the lower Yakima River valley, sets human health clean-up targets for DDT, and lays a path to achieve these targets. Ecology will work with the communities in the lower Yakima River watershed to continue to reach water quality goals in watershed.

Technical information

Unless otherwise specified, the following documents are Ecology publications.

Quality Assurance Project Plan: Lower Yakima River Temperature and Eutrophication Modeling, 2017

Quality Assurance Project Plan: Yakima River Side Channels Project, Temperature Monitoring and Water Quality Improvement

Project Update for the Lower Yakima River Suspended Sediment TMDL

Yakima River Pesticides and PCBs Total Maximum Daily Load: Volume 1. Water Quality Study Findings

Chlorinated Pesticides, PCBs, and Dioxins in Yakima River Fish in 2006: Data Summary and Comparison to Human Health Criteria

Lower Yakima River Suspended Sediment Total Maximum Daily Load: Effectiveness Monitoring Report

Lower Yakima River Suspended Sediment TMDL

A Suspended Sediment and DDT Total Maximum Daily Load Evaluation Report for the Yakima River

Related information

Unless otherwise specified, the following documents are Ecology publications.

Briefsheet - Lower Yakima River Cleanup Plan - a Plan Targeting Sediment and Pesticides

Focus Sheet: Suspended Sediment and DDT in the Lower Yakima River Watershed: Progress Update

WRIA 37: Lower Yakima Watershed Information (Water web site)


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Last updated August 2017
  Water resource inventory area (WRIA) 37 map, Washington State.


#37 (Lower Yakima)

Water-body Name:
Yakima River Basin - Lower Yakima River

Suspended Sediment

# of TMDLs:
DDT: 27*
Turbidity: 3

* the 27 DDT TMDLs were approved in 1997, but this approval was rescinded in 2004. See text for more details.

Approved by EPA

Contact Info:
Jane Creech
Phone: 509-454-7860
Email: Jane.Creech@ecy.wa.gov

Central Region
Department of Ecology
1250 West Alder Street
Union Gap, WA 98903-0009