BOR Flow Recommendation Changes Proposed, by Steve Harris, Harris Water Engineering

An important component of the recovery of the endangered Colorado Pikeminnow and Razorback Sucker in the San Juan River is the magnitude and pattern of flows in the critical habitat downstream of Farmington. The first development of the flows was in 1999 that  primarily focused on the quantity of water and timing of releases from Navajo Reservoir. Also in 1999, a range of equally important flow ranges were estimated to be beneficial to recovery of the fish: base flows of 500 to 1000 cfs; peak intermediate flows of 2500/5000/8000 cfs; and peak flow of 10,000 cfs or more. The outlet works at Navajo Dam cannot release more than 5,000 cfs so in order to obtain flows downstream of Farmington approaching 10,000 cfs, Navajo releases need to be matched with high Animas River flows (i.e. spring runoff). 

The 1999 flow recommendations used a “decision tree” that is based on Navajo content, projected inflow, and time since the last high release to determine potential spring peak release magnitudes (ranging from no release to a 3 week 5000 cfs spring peak release). The result of the “decision tree,” since the current drought began in the year 2000, has been that the Navajo content has been below average in most years due to the forced 5000 cfs peak release even in dry years.  Since 2000 there has been a threat nearly every year that shortages might occur to the fish base flow and water users below Navajo. One way to describe the past operation is the bottom half of Navajo was being released nearly every year and the top half was empty; a better operation would be to retain the storage in the bottom half for really dry years like 2002 and the top half of storage would be used in the “normal” dry years we have been experiencing.

A few important Navajo elevations and associated content are the water level when full is 6085’ with a content of 1,700,000 acre-feet (AF) and the minimum allowed level for the NAPI outlet is 5990’ with a content of 660,000 AF.  Navajo must operate between these two elevations.

Studies since 1999, indicate that the intermediate flows are less important than previously believed but the base and high flows may be more important. The “San Juan River Environmental Flows Workshop #1” was held in mid-February to consider an alternative method for determining how and when to make releases from Navajo that maximize base and high flows.  After much discussion at the workshop “an end of the water year target elevation of 6063’ (1,386,000 AF content) with an option to reduce to 6050’ (1,226,000 AF content) for biological flexibility” was agreed to. After accounting for fish base flow releases, water user needs, and having Navajo at 6063’ on October first, if there was any remaining water it would be used for a 5000 cfs release to attempt to match the Animas peak flow in order to reach 10,000 cfs in the critical habitat. 

By using this method, the top half of Navajo storage would be used most of the time saving the bottom half of storage for years when there is an exceptional drought. The threat of annual shortages to fish base flows will be significantly reduced and Navajo releases will maximize the high flows in the critical habitat. A secondary benefit is Navajo water levels will be higher for better lake recreation.