If you have ever fished Lower Lake Mary, you know it is a lake of boom and bust. Some years the lake is full and fishing is great and other years the lake is low or just a meadow. Well, the story of Upper and Lower Lake Mary is directly tied to the history of Flagstaff and its early settlers. Upper and Lower Lake Mary are named after Mary Riordan, the daughter of the wealthy lumber baron Timothy Riordan. The Riordans lived in the Riordan Mansion which is now an Arizona State Park.
Construction of the Lower Lake Mary Dam was completed by the Riordans in 1904 with a pipeline providing water to the town of Flagstaff. According to W.H. Power, “Sometime in the spring of 1905 great holes formed in the bottom of the reservoir on the west side of the channel about 1,200 feet from the dam.” From 1905-1907, “efforts were made to fill the large holes. A great many loads of rock, brush, and dirt were hauled and dumped into these holes. This, no doubt, stopped some of the flow of water, but the holes still remain and considerable water is lost through them. The only practical way of stopping this flow is to construct a levee around the holes, which would involve the moving of practically 10,000 cubic yards of earth at an approximate cost of $6,000.”
In 1907-1910, an attempt was made to mitigate seepage through the sinkholes and a large earthen levee was constructed between the dam and the sinkholes. Eventually, additional sinkholes formed on the lake side of the levee, negating its effectiveness.
Given the unreliable nature of the Lower Lake Mary water level, the Upper Lake Mary Dam was constructed in 1941 to provide a more permanent water supply for the City of Flagstaff. Today, we better understand the geology of the area surrounding Lower Lake Mary and know that it sits on a layer of Kaibab Limestone and two fault lines, the largest being the Lake Mary Fault (Natural Channel Design Feasibility Study 2018). These are the main reasons why Lower Lake Mary fails to be a permanent lake.
The natural ebb and flow of water levels in Lower Lake Mary are not necessarily a bad thing for fisheries managers. The last time Lower Lake Mary filled and spilled was during the 2005-2006 winter season, but Lower Lake Mary has held enough water to be a fishery 16 out of the last 20 years.
During years when the lake is low, dry terrestrial vegetation in the form of grasses thrive in the Lower Lake Mary Basin. When the lake fills, these grasses are inundated with water where the nutrients from the newly flooded decomposing vegetation are released into the lake, increasing primary productivity.
At the same time, earthworms emerge from the newly flooded lake bed, providing an immediate food source for freshly stocked fish such as rainbow trout. As the spring transitions into summer, aquatic vegetation readily grows, providing the necessary habitat and food for aquatic insects such as midges, caddisflies, dragonflies, and damselflies, which are all food that fish love. Duck leeches also readily become established as waterfowl take advantage of the new lake. The leeches provide yet another food source for the fast-growing trout.
With all this new food available in the highly productive new lake, the stocked trout grow faster than we can achieve in many of our hatcheries at a rate of about one inch per month. During years when Upper Lake Mary fills and spills, fish species that are resident in Upper Lake Mary, such as northern pike, spill over into Lower Lake Mary and often have a successful spawn. If the lake persists for at least two years, a mix of stocked trout and species that are resident in Upper Lake Mary are available for anglers to enjoy. This frequent drying and refilling of the lake allows the system to reset each time.
The winter of 2022-2023 was the third-snowiest on record for Flagstaff, with 163.1 inches of snow reported at Flagstaff’s Pulliam Airport. With all of the snow, a significant amount of runoff was inevitable. The AZGFD Flagstaff Region staff watched in eagerness and amazement as Upper Lake Mary went from about 24% full during the fall of 2022 to completely full and spilling by March 22, 2023.
Upper Lake Mary poured into lower Lake Mary, and by April 5, Lower Lake Mary was completely full and starting to spill into Walnut Canyon. Eager to get fish into the lake, AZGFD staff worked with the Coconino National Forest and Coconino County to gain access to the lake for stocking. On March 24, the county removed large snow drifts blocking the roadway into the day use area of Lower Lake Mary, providing access to the boat launch. AZGFD took advantage of the newly acquired access and the full lake by stocking 67,222 rainbow trout from April 5-7.
However, the spring runoff wasn’t done yet. The Lower Lake Mary water level continued to rise, and on April 10, the levee was breached and Lower Lake Mary reclaimed about 35 surface acres of its historic lake bed. The breach was most likely caused by the resident prairie dog colony that had burrowed tunnels throughout the levee over the past 100+ years.
Current concerns over the breach failure turned out to be a moot point as lake levels continued to rise, and on April 12, the lake flowed over the levee, and the portion of the lake that was flooded by the breach would have flooded anyway. We will have to wait to see what the long-term effects of the levee failure are as the water level drops.
The lake levels have risen so much that the picnic tables and most of the parking lot of the day use area are underwater. Even the dumpster normally positioned near the boat launch was recently observed freely floating around the lake. Currently, the lake continues to spill and rage down Walnut Canyon at an impressive rate.
Every year is different, and we have come a long way from the drought and wildfires that plagued us during last year. This year is one for the record books.
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