Gene's Corner: Lake Restoration/Rehabilitation by Dilution

Written October 2024, by Gene Welch, Professor Emeritus, Civil and Environmental Engineering, University of Washington

Moses Lake has been diluted with low phosphorus Columbia River Water (CRW) for 47 of the past 48 years, ever since the Clean Lakes Project started in 1977. Regular annual inputs of CRW have ranged from about 60,000 to 320,000 AF (acre feet) and has averaged about 200,000 AF the past 24 years. That average inflow equals about 1.6 lake volumes. During those 24 years, May to September average total phosphorus concentration (TP) at lower Parker Horn and South Lake averaged 25 ppb (parts per billion by weight). That low TP concentration included algae, detrital particles, and soluble P (even DNA).

Without added low-TP CRW with only 8 ppb, the inflow TP concentration to the lake from Rocky Ford Creek, Crab Creek and ground water would have been 109 ppb instead of 33 ppb with CRW, as averaged from the May-September TP loading analyses in 2020 and 2021. The predicted whole-lake TP concentrations would have been 168 and 50 ppb without and with CRW, respectively, including input from bottom sediments. Observed whole-lake (all depths) TP was 55 ppb, similar to the predicted 50 ppb. Average surface TP at six lake sampling sites was 27 ppb, the concentration that relates to algae and represents the lake’s quality. So, low-TP CRW predictably lowered lake TP by diluting inflow TP concentration as well as the input from bottom sediments.

Rehabilitation of lakes by dilution is not unique to Moses Lake. Lake Spokane water quality has markedly improved since 1977 when phosphorus removal was installed in Spokane’s wastewater that is discharged to the river. As a result, the river TP concentration entering the lake was reduced from 86 ppb to 25 ppb and eventually to 14 ppb. Treated city wastewater now contains TP at 9-16 ppb. So, the lake’s rehabilitation was due largely to dilution. Same is the case for Lake Washington. Diversion of 9 wastewater effluents from the lake during 1964-1967 lowered lake TP from 64 to 15 ppb in four years. That quick recovery was largely due to the low TP (17 ppb) in the Cedar River, the main inflow which diluted the lake’s high TP concentration. Also, water quality in Green Lake in Seattle was effectively improved by dilution with Seattle city water containing only 6 ppb TP. So, dilution has been more prevalent in lake rehabilitation than often recognized.

This rehabilitation method has been usually referred to as dilution/flushing. However, the flushing part is much less relevant to the reduction in lake TP than dilution. A large increase in inflow will increase the lake water exchange (replacement) or flushing rate. The average CRW inflow over the past 25 years of 200,000 AF during April through September increased the lake flushing rate from about 0.25% per day to 0.9% per day. That increase in flushing rate will not reduce lake TP concentration unless the inflow TP concentration is lowered, as occurs with CRW at 8 ppb TP.

The rehabilitation process for Moses Lake was initially described as eutrophication control by flushing back in 1968, but was clarified to lake water users as dilution before the Clean Lakes Project could begin. The lake outflow will be larger with an increased rate of water exchange (flushing), but with a much lower (diluted) TP concentration in the lake outflow. So, while there is more phosphorus leaving Moses Lake with the high CRW inflows, because the CRW contains some phosphorus too, the outflow TP concentration is lower at about 25 ppb instead of well over 100 ppb.

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