Bumper crops of various aquatic insects sometimes occur due to water quality changes such as temperature, nutrients or even chemical spill that wipes out habitat for others.  Even excess salt from roads.  The reduction in predation another possibility from trout.  Sometimes the conditions change just enough to favor a certain species.  From memory, Colorado has had some   pretty severe disturbances relative to storms and wildfires, so perhaps this series is an invader after disturbance, so its population will level off with more competition.  I am not familiar with this specific species to suggest more, but do remember instances when the front of the car and windshield was covered with caddis fly victims.    

I am very familiar with this species in Texas.  It definitely has a preference for disturbed streams, and it can reach enormous population densities.  Part of the increase you are seeing may be a natural population fluctuation.  On the other hand, the increase could be due to disturbance on the Colorado, especially any disturbance that increases the seston load that would serve as food for this filter-feeding species.  My guess would be increase seston load is due to increasing nutrification that is causing enrichment. Increased bacterial loading could also be involved.  Some additional things to consider:  1) Has the increase in population  size been associated with drought (reduced flows will favor this species, or at least concentrate them), and   2) Do the large increases occur above or below inpoundments?--that may help you better estimate the cause.

Thank you both for your responses. This problem is occurring just downstream from Davis Dam. I think they did experience a drought a few years ago, so seston could have built up in Lake Mohave above the dam. There are two cities here, one does not empty its treated effluent into the river, and the other does. The effluent that goes in has actually decreased over the last decade or so due to a reduction in tourism. That leads me to believe that bacteria may not be an issue. On the other hand, there have been many developments of park land next to the river. They use an abundance of fertilizer to keep the grass green. One thought is that the fertilizer is getting into the river, causing the growth of algae, and feeding the caddisflies. The fertilizer issue may be resolved by creating a buffer area with plants to help soak up the nutrients before they get to the river. I'm not sure what can be done about the nutrient load from the dam release. Thank you both for your input.

I think David hit the nail on the head.....seston.  Any thing leading to higher particulate loading tends to increase hydropsychid abundance.  We see them in large numbers in our most urban streams where impervious surfaces lead to increased bank and bed erosion.  They also respond positively to agriculture.