OAR 340-056-0400
Flow Determination Methodology


The Department will determine the instream flow need for pollution abatement based on stream specific analysis, using the methodology described below for which there is adequate data, and which, in the Department’s judgement, is most appropriate and reliable.


The flow determination methodologies described below are based on existing users, including facilities with discharge permits and existing land uses that generate nonpoint runoff or otherwise impact the water quality of the stream.


Available and relevant data on streamflow, water quality and pollution loading will be compiled to determine the appropriate methodology and water quality parameters to be used in making the instream flow need determination.


The methodologies the Department will use to determine the instream flow need for pollution abatement include, but are not limited to, the following:


Streamflow — Water Quality Correlation Analysis: Available data is analyzed to determine whether a correlation exists between observed water quality and instream flows. The streamflow condition at which water quality problems are minimized is identified. This analysis accounts for point source, nonpoint source, and background loads affecting the stream segment at the time the water quality data were collected;


Load Assimilation Analysis: Information on pollution loads is used to estimate or predict the flows needed to assimilate the loads in the stream. Dilution rule requirements or simplified water quality models, based on available data, are used to determine the flows needed to assimilate the permitted effluent discharges (point sources) and estimated nonpoint source loads to the stream. If the permitted discharges do not meet the minimum design criteria for treatment and control of wastes specified in the basin rules (OAR 340-041) at the time of the instream water right application, the Department may estimate the instream flow need that will occur when the facility upgrades to the minimum design criteria and base the water right application on that flow level;


Water Quality Modeling: Special intensive stream investigations, such as those conducted in basins receiving Total Maximum Daily Loads (TMDLs) provide detailed data on instream water quality, point and nonpoint source loads, and streamflows. When this data is available, it will be used to model the relationship between streamflow and water quality, and estimate the instream flows needed to assimilate permitted effluent discharges, estimated nonpoint source loads, or other factors affecting water quality, such as temperature. In TMDL basins, dischargers likely have to meet more stringent discharge limits than basin design criteria require. These facilities may have permit limits for additional parameters than are included in a typical permit;


Non-Degradation Flows Method: This methodology applies only to Outstanding Resource Waters subject to the non-degradation standard. For these waterbodies, the Department will strive to maintain existing water quality and, therefore, existing streamflows. Any reduction in streamflow could potentially contribute to a degradation of water quality. A simple method to achieve this objective is to apply for an instream water right for the median monthly streamflow or lake levels by month.

Source: Rule 340-056-0400 — Flow Determination Methodology, https://secure.­sos.­state.­or.­us/oard/view.­action?ruleNumber=340-056-0400.

Last Updated

Jun. 8, 2021

Rule 340-056-0400’s source at or​.us