Herrera also prepared a summary of existing data:

Sporadic monitoring of Curlew Lake has occurred since the 1980s. The most intensive monitoring was conducted from 1986 to 1987 through the Washington State Water Resources Center, Washington State University, and the University of Washington (Juul et al. 1988). This study collected a substantial amount of data on lake and stream water quality, hydrology, lake plankton, macrophyte community, and lake sediment. The report’s data has not been digitized and is not incorporated into this summary. Juul et al. (1988) concluded that the lake was mesotrophic (moderately productive) and that resulted from “(1) occasional high water levels flooding near-shore septic systems, (2) livestock grazing adjacent to inlet streams, (3) past timber harvesting practices, and (4) a groundwater source that has a high concentration of phosphorus in some areas.” Juul et al. (1988) recommended the following to preserve and restore water quality: (1) non-point source control (e.g., livestock), (2) harvesting of non-game fish, particularly northern pikeminnow, to remove nutrients, (3) protect and restore wetlands to act as nutrient sinks, (4) construct a dam at the outlet to control lake levels and allow winter drawdowns, and (5) implement and enforce development zoning ordinances. Juul et al. (1988) did not recommend in-lake nutrient management through alum, hypolimnetic aeration, or dredging because of the large cost of those approaches.

In the 1990s, the lake was monitored twice nearly every year, and a more thorough investigation of the lake, its inlets, and its outlet was undertaken in 2010 and 2011. However, a formal report was not prepared for the 2010-2011 study.

There are no discernable long-term trends in the water transparency and surface phosphorus concentration in the lake, but there is limited evidence that phosphorus levels in the hypolimnion have increased (Figures 5 and 6). The data available suggest that Curlew Lake is borderline mesotrophic/eutrophic, and phosphorus levels are substantial in hypolimnion (>100 µg/L). Available profile data show that the lake is strongly stratified in April with a thermocline at approximately 5 meters (Figure 7). Thermal stratification appears to remain strong through October and weakens, but does not disappear, in November. No lake monitoring has occurred between December and March while the lake is partially or completely frozen over. We anticipate that the lake undergoes a brief mixing event in November or December before becoming covered in ice and becoming winter stratified, with ice and less dense water at less than 4 degrees Celsius in the epilimnion and more dense water at 4 degrees Celsius in the hypolimnion. With thaw out in March or April, there is another brief mixing event before stratification begins again.

Monitoring data from the 2010s suggest that phytoplankton productivity in Curlew Lake is typically phosphorus-limited and occasionally co-limited by phosphorus and nitrogen. Total N:P ratios together with dissolved nutrient concentrations can be used to indicate which nutrient is most limiting to algae growth. Guildford and Hecky (2000) found that ratios greater than 22 indicate phosphorus limitation, ratios less than 9 indicate nitrogen limitation, and ratios between 9 and 22 indicate co-limitation of algae growth by both phosphorus and nitrogen. The elevated dissolved inorganic nitrogen concentrations (about 50 µg/L) and dissolved orthophosphate phosphorus concentrations (about 8 ug/L) observed in the surface waters of Curlew Lake in 2021 (Ecology 2022) together with a total N:P ratio of around 35 (greater than 22) suggests that algae growth in Curlew Lake is limited primarily by phosphorus.