waskington state lake images
WATERLINE - August, 2023

Diluting Moses Lake to improve water quality since 1977

by Dr. Gene Welch, Limnologist and Professor Emeritus, University of Washington
and Shannon Brattebo, PE, Environmental Engineer, Tetra Tech, Inc.

Figure 1. Sampling sites by MLIRD during 2017-2021. Most sites are similar to those sampled by UW Civil and Environmental Engineering during 1969-1970 and 1977-1988 (Welch et al., 1989).

Renewed attention to the water quality of Moses Lake began in 2019 when high concentrations of microcystin were detected in nearshore algal scums. The Moses Lake Irrigation and Rehabilitation District (MLIRD) had already restarted — in 2017 — the systematic water quality program they had supported in the 1970s-1980s (Welch et al., 2019). The detection of toxic algae prompted formation of the Moses Lake Watershed Council to help develop support for recommendations to improve the lake’s quality. There is a long history of study and projects that led to improved lake quality from the 1960s when more than 90% of algal biomass was Aphanizomenon and total phosphorus (TP) was 154 µg/L. The lake improved from hypereutrophic to eutrophic through the 1980s and further to near-mesotrophic through the 2000s.

History of restoration
In the 1960s, the MLIRD engaged faculty and students from University of Washington to study the lake and propose remediations. That effort resulted in a proposal to increase the input of low-nutrient Columbia River water (CRW) through existing infrastructure. That method had improved water quality in Seattle’s Green Lake by adding low-phosphorus Seattle drinking water.

The ability to deliver low-phosphorus CRW (5 to 8 µg/L) to Moses Lake was present but sparingly used until the Clean Lakes Project began in 1977, when the US Bureau of Reclamation increased inflows, usually during spring through early summer. Inflows were modest in the 1970s-1980s, but increased in the 1990s. They ranged from 101,000 to 203,000 acre-feet (AF)–(125 to 250 106 m3) over the next 47 years. Those inflows represent 0.8 to 1.6 lake volumes and lake water exchange rates of 0.5 to 1% per day.

Thus, the process that has improved lake quality in Moses Lake is a more gradual dilution rather than a visible exchange of water or flushing. The addition of water with very low phosphorus has diluted phosphorus in the lake. That results in water leaving the lake entering Potholes Reservoir with a reduced phosphorus concentration–and inflow concentration determines lake water quality. Therefore, diluting Moses Lake improved water quality downstream. For example, the average May-September TP concentration leaving the lake in 2021 and 2022 was 20 µg/L, due to 230,000 and 187,000 AF of CRW inflows, respectively. Without the CRW inflows, the TP leaving the lake would be around 200 µg/L, as predicted from TP in surface inflows, groundwater, and internal loading, the latter of which was 50% of the total in 2020 and 53% in 2021 (Tetra Tech 2021 and 2022).

Figure 2. Inflow of Columbia River water into Moses Lake over 52 years and spring-summer average TP concentration in Lower Parker Horn and South Lake.

Lake quality history
Before the Clean Lakes Project, during 1969-70, average May–September TP in the lower lake half (Parker Horn, South Lake and lower Pelican Horn; Figure 1) stood at 154 µg/L, and ultimately improved to 44 µg/L during 1986-1988, following wastewater diversion in 1984. Average chlorophyll decreased from 57 to 17 µg/L and transparency increased from 0.8 to 1.6 m (Welch, 1992). There were further improvements over the past 23 years with an increased average CRW inflow of 203,000 AF and an average lower lake half TP of 25 µg/L (Figure 2). Total P was even lower in the lower lake half during 2021 and 2022 at 20 µg/L with a transparency of 3 meters and CRW inflow of 230,000 and 187,000 AF, respectively.

Total P was usually higher in the upper lake half, Rocky Ford Arm, which is fed by Rocky Ford Creek with exceptionally high TP (160 µg/L), most of which is soluble. However, TP in Rocky Ford Arm in 2021-2022 was lower than in the past at just 36 µg/L with a transparency of 1.9 m.

Blue-green algal biovolume fractions were much lower in 2021 and 2022 than in the previous four years; at 22% and 12% in the lower lake half and 24% and 36% in Rocky Ford Arm. Blue-greens, also referred to as cyanobacteria, were also low in the lower lake half in 2001 at 5% when CRW inflow was also high at 230,000 AF (Carroll, 2006). The low blue-green fraction was consistent with a low TP of 19 µg/L. Microcystis was usually the dominant blue-green the past six years, while Aphanizomenon dominated in 1968-1970, before dilution began, and through 1988 (Welch, 1992). The dominance switch may be due to lower TPs resulting in observed higher TN (total nitrogen):TP ratios.

Figure 3. Relation between May-September average TP at South Lake and lower Parker Horn and % Columbia River water at 0.5 m from 1977 to 2022. The point at 10%CRW and 152 µg/L TP was pre-dilution in 1969-1970.

Distribution of Columbia River water
The inflow of CRW enters Upper Parker Horn via Rocky Coulee Wasteway and Crab Creek. The inflow was shown to distribute throughout the lake early in the Clean Lakes Project, even well up into Rocky Ford Arm (Welch and Patmont, 1980). The distribution was traceable by specific conductance, which is low in CRW (142 µS) and relatively high in Crab Creek (491 µS) and Rocky Ford Creek (381 µS; Welch and Patmont, 1980). The percent of lake water and CRW in the lake, calculated using an equation developed by Welch and Patmont (1980), has shown that there is typically a very high percentage of CRW in the lower lake half and less CRW in Rocky Ford Arm. However, recently there has been a higher percentage of CRW reaching into Rocky Ford Arm. Fractions of CRW reached 79% and 75% in the lower lake and middle and lower Rocky Ford Arm during July-September in 2021 and 2022 with total spring-summer CRW inflows of 230,000 and 190,000 AF, respectively. Fractions of CRW were less in 2020 at 66% CRW, although total CRW inflow was similarly high as in 2022 at 187,000 AF. Average May-September percent CRW has been related to average TP in the lower lake half since the start of the Clean Lakes Project (Figure 3).

The greater dilution effect seen in 2021 and 2022 may be due to the extent of lake level drawdown before CRW inflow begins around April 1 each year. Pre-CRW drawdown in 2021 and 2022 averaged 2.3 feet lower than in 2017 and 2019. That 2.3-foot difference represents 19% of full pool volume. Less volume in the lake at the start of dilution decreased the ratio of lake volume to CRW inflow and would have increased the % CRW in the lake. That may partly explain the more effective dilution–higher % CRW–in 2021 and 2022, and consequently lower lake TPs. However, the amount of CRW inflow is not guaranteed each year and can vary significantly. Inflows of CRW averaged much lower during 2017-2019 at 98,000 AF and CRW was only 56%. Inflows depend to some extent on the amount of natural runoff and the water needs of downstream irrigators (Welch et. al., 2019).

Blue-green algae bloom at Blue Heron Park boat launch (date unknown) (Photo Provided by: Moses Lake Irrigation and Rehabilitation District)

Recent lake water quality
Lake water quality conditions were greatly improved in 2021 and 2022 over the previous four years. Total P averaged 20 µg/L during May-September in both years in the lower lake half and 36 µg/L in Rocky Ford Arm. Chlorophyll averaged 7.5 and 11 µg/L in the two lake areas, respectively, and transparency was 3.0 and 1.9 m. In the previous four years (2017-2020), average TP was 34 µg/L, chlorophyll 14 µg/L and transparency 1.5 m in the lower lake half, and 86 µg/L, 44 µg/L and <1 m for those constituents in Rocky Ford Arm (referred to hereafter as RFA). As previously noted, improved lake quality in 2021 and 2022 was due to more CRW inflows, as well as a greater ratio between CRW inflow and pre-inflow lake volume.

Moses Lake looking toward Pelican Horn and South Lake (date and photographer unknown) (Photo Provided By: Moses Lake Irrigation and Rehabilitation District)

Phosphorus internal loading
The morphometry of Moses Lake favors high rates of recycled phosphorus from bottom sediments as internal loading. The large lake area of 2,770 hectares (ha) and relatively shallow mean depth (5.6 m) gives an Osgood index of 1.1. Values <6-7 indicate a strong tendency for wind mixing to entrain bottom anoxic water with high phosphorus. Dissolved oxygen concentrations at 0.5 m off-bottom in the lower lake half and lower RFA averaged 3.8-4.6 mg/L during the summers of 2020-2022. Anoxic sediment-P release determined in sediment cores collected from the lake was 10 mg/m2 per day (Okereke, 1987). Such high release rates are typical from sediment overlain with anoxic water, which occurs even in shallow lakes. For example, Upper Klamath Lake (OR) has a mean depth of 2.4 m, an Osgood Index of 0.1 and low off-bottom dissolved oxygen during low wind speeds. Those conditions produced net internal loading of 6 mg/m2 per day (Kann and Welch, 2005; Welch and Cooke, 1995).

Internal loading in Moses Lake during 1984-1988, after wastewater diversion, averaged 9,346 kg or 43% of total loading during May-September (Jones and Welch, 1990). That load represented a release rate of 3.4 mg/m2 per day that included 27% of the lower lake half and middle and lower RFA area over 5 m, as well as the remaining shallower area that is usually oxic (oxygenated) with a release rate of 1 mg/m2 per day (Okereke, 1987). That would account for the average whole-lake rate at much less than the anoxic rate. Internal loading is still high despite dilution with large CRW inflows. Lake TP has tended to increase as summer progresses on the order of 40-60% in the lower lake half and middle-lower RFA. Those increases resulted in internal loading of 8,318 kg and 10,281 kg in 2020 and 2021, or 49.9% and 55.3% of total loading, respectively (Tetra Tech, 2020 and 2021). Nevertheless, the high CRW inflows in 2021 and 2022 still resulted in the high percentage of CRW and low average TPs (Fig. 3). Those high internal loading rates indicate that lake TP can be further lowered by sediment-P inactivation.

Blue-green algae bloom in Moses Lake in Lower Park Horn above I-90 Bridge in 1968. (Photo Credit: Eugene Welch)

Moses Lake has a long history of improved quality due to increased inputs of low-phosphorus CRW, shown through monitoring its changing condition. The data show that lake TP concentrations in the lower lake half decreased by dilution with increased inflow of CRW, from a pre-dilution concentration around 150 µg/L to about 20 to 30 µg/L. The lake’s quality improved over the first 12 years of the Clean Lakes Project from being hypereutrophic to eutrophic through the 1980s to near-mesotrophic (<25 µg/L) through the 2000s. Lake quality improvement, in terms of reduced TP, was proportional to CRW inflow volume and resulting percentage of CRW in the lake.

Effects of the inflows of low-phosphorus CRW are best described as dilution rather than flushing, because water exchange rates ranged from only 0.5 to 1% per day. Without dilution, whole-lake TP would be around 200 µg/L, largely due to half the phosphorus loading coming from internal recycling. Results further show that if TP is on the order of 20-25 µg/L, blue-green biovolume will be relatively low and transparency will be on the order of 3 m. This was the case in 2021 and 2022, when the lake had outstanding water quality due to large amounts of CRW, and more importantly, higher percentage of CRW throughout the water column. As mentioned previously, the amount of CRW available for dilution each year is not guaranteed, and internal phosphorus loading continues to be a large fraction of total loading, even with large amounts of CRW.


Carroll, J. 2006. Moses Lake phosphorus-response model and recommendations to reduce phosphorus loading. Pub. No. 06-03-011. Washington Dept. of Ecology, Olympia, WA. 

Jones, C.A. and E.B. Welch. 1990. Internal phosphorus loading related to mixing and dilution in a dentritic, shallow prairie lake. Journal of the Water Pollution Control Federation. 62: 847-852.

Kann, J. and E. B. Welch. 2005. Wind control on water quality in shallow, hypereutrophic Upper Klamath Lake, Oregon. Lake and Reservoir Management. 21:149-158.

Okereke, V.O. 1987. Internal phosphorus loading and water quality projections in Moses Lake. M.S.E. Thesis. Civil and Environmental Engineering, University of Washington, Seattle, WA.

Tetra Tech, 2021. Moses Lake – Whole Lake Mass Balance Model & Management Alternatives Evaluation. Technical Memo prepared for Moses Lake Irrigation and Rehabilitation District. MLIRD | Water Test Results

Tetra Tech, 2022. Moses Lake Water and Total Phosphorus Budget for 2021. Technical Memo prepared for Moses Lake Irrigation and Rehabilitation District.

Welch, E.B. 1992. Lake trophic state change and constant algal composition following dilution and diversion. Ecological Engineering. 1:173-197.

Welch, E.B., S.K. Brattebo, and C. Overland. 2019. Four decades of diluting phosphorus to maintain lake quality. Water Environment Research. 92:26-34.

Welch, E.B. and G. D. Cooke. 1995. Internal phosphorus loading in shallow lakes: Importance and control. Lake and Reservoir Management. 11:273-281.

Welch, E.B. and C.R. Patmont. 1980. Lake restoration by dilution: Moses Lake, Washington. Water Research. 14:1317-1325.