Shagawa Lake was formed during the last ice age approximately 10,000 years ago. The lake has 3 relatively deep (~12 m deep) basins but has a primarily flat bottom at 6-8 m with abrupt sides and a moderately rough shoreline. The lake has many islands and submerged outcroppings occur throughout the lake. The Burntside River (from oligotrophic Burntside Lake to the west) enters the western end of Shagawa and accounts for about 70% of the yearly inflow into the lake. Another 20% inflow comes from several minor tributaries that drain the surrounding forested lands. There is one outlet, the Shagawa River, located at the eastern end of the lake. The wastewater treatment plant is located midway along the southern shore (Larsen and Malueg 1976).

In 1973, advanced treatment to remove >75 % of the phosphorus from the Ely Municipal Wastewater Treatment Plant's discharge began. Although significant water quality improvement was noted in the late 1970's (summertime in-lake P declining by 35-50%) relatively high levels of algal chlorophyll persisted at levels higher than predicted. The reason is that lake bottom sediments retained an enormous reservoir of phosphorus from previously high loadings. This sediment P is released each summer into the hypolimnion when this bottom water layer becomes depleted in oxygen (anoxia). Phosphorus can then diffuse out of the sediments and the hypolimnion becomes greatly enriched with this nutrient. This presents no problem until it becomes very windy and some of this deeper water is mixed into the upper epilimnion (upper 3-10 m water layer) and at the onset of fall overturn in late September or October. Algae are then fertilized and grow faster leading to potentially nuisance-level blooms and loss of water clarity, as indicated by decreased secchi depth.

This situation continues to the present time and the lake will take many decades to fully recover (if ever). Some of the released phosphorus becomes permanently bound into the sediments each year, but this is "balanced" by new inputs of P from the treatment plant (a point source discharge) as well as other non point sources from the watershed (on-site septic sysytems, road and shoreline erosion, lawn fertilizers etc). Even with the state of the art P-removal from wastewater treatment plants, the concentration of P in the discharge will always be at least 10 times higher than natural streams, and the total load will increase as the city grows.

Shagawa Lake (~May through Sepetmber)
Years	Trophic Status Index (TSI-M)	Secchi Depth	Trophic Status
early 70's	>60	??	eutrophic
1979-1981	56	~1.8 m	eutrophic
1988, 1992-94	54	~2.2 m	meso-eutrophic
1996, 1998, 2000	~48	~2.5 m	mesotrophic
NLF Ecoregion	~44-51	2.4-4.6m	mesotrophic