Oregon Administrative Rules › Chapter 340 - DEPARTMENT OF ENVIRONMENTAL QUALITY › Division 56 - INSTREAM WATER RIGHTS › Section 340-056-0400 - Flow Determination Methodology

Current through Register Vol. 63, No. 7, July 1, 2024
(1) 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.
(2) 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.
(3) 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.
(4) The methodologies the Department will use
			 to determine the instream flow need for pollution abatement include, but are
			 not limited to, the following:
(a) 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;
(b) 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;
(c) 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;
(d) 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.