Crappie populations are extremely cyclic - that is to say, usually one or two year classes will dominate a population until they get old and die and leave room for another year class to survive and thrive. After several years of fishing on two very strong year classes, it appears they have passed from the "picture" in Brownlee and Oxbow.
The other issue on Brownlee Reservoir is drawdown. Crappie are highly susceptible to extreme drawdown and will be swept from the reservoir. Because of water conditions in 2012, Brownlee Reservoir was drawn to a very low elevation. Many of the crappie that would have been recruited to the fishery in 2013, were entrained through the dam and probably didn't survive.
In reality, the lack of crappie in the 2013 fishery was probably a combination of the two factors. Hopefully, Brownlee Reservoir pool will be held at a stable level in the spring of 2014 and allow for successful spawning and recruitment of a new, strong year class that will show-up in two or three years.