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Technical Report

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Illinois river watershed, water quality, phosphorus, algal species


The “Joint Study” was conducted to fulfill the obligations of the second “Statement of Joint Principles and Actions” agreed to by the states of Arkansas and Oklahoma. The “Joint Study” affirmed the magnitude of Oklahoma’s Scenic River total phosphorus (TP) criteria (i.e., 0.037 mg/L), but it added the new caveat of applying the criteria to “critical conditions.” The primary purpose of this paper was to define “critical conditions” based on the range in base flow proportions (BFP) of total streamflow on days that were sampled in the “Joint Study,” where BFP is base flow discharge divided by total stream flow for a given site and sampling date. We focused on 20 stream sites that could be paired with USGS stream discharge monitoring stations where water samples were collected approximately 12 times over the two-year “Joint Study” (June 2014–April 2016). In fact, 93% of the water samples from the “Joint Study” used to measure TP concentrations were collected when base flow contributions were 80 percent or more of total stream flow (i.e., BFP greater than or equal to 0.80). A subset of these sites in northwest Arkansas have been monitored more frequently between 2015 and 2019, and data from these sites (plus one additional urban stream) was used to evaluate the relation between TP concentrations and BFP. Across all sites, TP concentrations decreased as a function of increasing BFP – that is, TP concentrations were less on average as the proportion of base flow discharge increased at each site. The change in TP concentration per 0.1 unit change in BFP was positively correlated to mean TP concentrations when BFP was greater than 0.80. Defining the appropriate hydrologic conditions to assess the magnitude of the Oklahoma Scenic River TP criteria (0.037 mg/L) definitely matters for streams with TP concentrations approaching 0.037 mg/L during “critical conditions.” For example, if three water samples were collected at BFPs of 0.80, 0.70 and 0.60 with TP of 0.037 mg/L during “critical conditions,” then the mean of those three samples could [theoretically] be 0.045 mg/L (exceeding the TP criteria magnitude). Thus, if the TP criteria was going to be applied outside the hydrologic conditions studied, it should be adjusted based on the relation between TP concentrations during “critical conditions” and change in TP concentration per 0.1 unit change in BFP.


Funded by the Northwest Arkansas Regional Planning Commission

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