Annual Temperature, Precipitation and Flow
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Annual Temperature Data  
Temperatures on the San Juan River are moderate compared to many other trout fisheries providing fishable weather nearly 365 days a year.  The warmest month of the year is usually July with average highs around 90° F and overnight lows around 60° F.  The coldest month is typically January with average highs of 39° F and average lows around 18° F.  Modern fishing gear can make you comfortable fishing in practically all weather conditions.  Due to the high altitude, it is strongly recommended that you wear clothing and sunscreen to prevent sunburn.  River temperatures are relatively constant year round due to the deep release from Navajo Dam providing consistent insect activity throughout the year.  Hyperlink to River Stats page

Average monthly highs are shown in red and average monthly lows are shown in blue. 
Annual Precipitation Data  
The San Juan River is located in the High Desert, averaging  about 13" of precipitation per year.  Most months average about 1" of precipitation with June being the driest month (0.6") and August being the month with the most precipitation (1.7").  Summers generally bring thunderstorms, usually scattered and of relatively short duration.  During these thunderstomrs lightning can be present and anglers should take actions to reduce their risk.  These storms are generally short in duration but may be high in intensity.  If you are lucky enough to be on the river early in summer after the first heavy rain you might get to experience a tremendous fishing day as high numbers of ants are washed into the river providing some exciting dry fly fishing.   
Average monthly precipitation is a little over 1 inch per month.  June is the driest month while August is the wettest.
San Juan River Flow  
Flows in the San Juan River are influenced by many factors and are managed by the Bureau of Reclamation.  Many different parties have an interest in the water from the San Juan and a compromise plan has been put together to best meet the water demands of competing interests.  Current and future flow will be guided by the Flow Recommendations for the San Juan River, a plan developed by federal, state and tribal agencies and private groups.  One of the major influences for flow is an attempt to manage flows similar to historical values with high flows in spring.  These high flows in spring are thought to benefit endangered fish species in the lower San Juan River protected by the Endangered Species Act  

San Juan Flow 2014
Flows change from year to year as the Bureau of Reclamation must manage the available incoming flow versus demands downstream. 

The year 2008 was a high water year with sustained high flows from February through most of June.  During high flows, wading fishermen should take extra precautions.  Drift boats are able to fish the high flows well.

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