Grand Lake, Colorado

Since 2010, Hydros has been developing annual reports for the Three Lakes System for Grand County, Northern Water, and U.S. Bureau of Reclamation. The purpose of these reports is to assess observations of water clarity, dissolved oxygen, and chlorophyll a each year in the context of operations, meteorological conditions, hydrology, and nutrient loading.  Data extending back to 2007 are considered in each evaluation to support the development and testing of cause-and-effect relationships, as well as providing historical context. Additionally, a custom water-quality index (WQI) developed by Hydros for the Three Lakes System is applied to provide a numeric index summarizing overall water quality and supporting comparison across years of record.

These annual reports have led to a deeper understanding of operational control options for this unique system. For example:

• Summer Farr pumping (which pumps water from Granby Reservoir to Shadow Mountain Reservoir and causes water to then flow into Grand Lake) can prevent anoxia/low dissolved oxygen concentrations at the bottom of Shadow Mountain Reservoir by providing a source of well-oxygenated water to the bottom of the reservoir during the summer months.

• Summer Farr pumping can also affect reservoir water temperatures in Shadow Mountain Reservoir, which can reduce nuisance algae growth and improve clarity. There appears to be an optimal target temperature range for Shadow Mountain Reservoir that is cool enough to limit summer growth of Dolichospermum, yet warm enough to limit summer growth of Asterionella, as occurred in 2018 and 2019.

• Summer Farr pumping tends to reduce clarity in Grand Lake since Shadow Mountain Reservoir often has poorer clarity than Grand Lake. Therefore, optimizing clarity in Shadow Mountain Reservoir through careful management of summer Farr pumping, when possible, is key to managing clarity in Grand Lake.