It all comes down to the Ecosystem.The Delaware system can be complex.For an example the upper West Branch there is without a doubt some big changes over the years i.e. Species disappearing ,less prolific hatches, and hatches appearing that where never seen before like the Green Drake.I always say it's all about water temps.However in this case it is silt as the major culprit, and less oxygen levels and I'm sure there are other factors.Than there is PH levels and minerals and maybe chemicals.The only way this can be tested properly is you would need a baseline from years ago to compare-nobody ever did the research.
On the Flip side there is a section of a certain river that hatches have become more prolific, and just this past summer I had consistent fishable summer Steno hatches from Mid June until mid September.In over 35 plus years I have never seen this before during the summer.Why? I really don't know without science.My theory is that the previous fall, winter, and early spring there was a steady significant amount of water in the river about 5x the Norm.Is that the answer no clue.
In summary hatches are dictated by water temps,minerals, silt, PH levels,chemicals in the water?, Oxygen levels, etc.Without Science you will never know the real reasons to answer the question- WHY?
