The way I understand it is both lakes and streams have hatches of midges which are all chronomids. The hatches in rivers are usually of the smaller variety than what you would get in a lake. In lakes where they are found they can make up to 90% of a trouts diet.
Eastern Washington and BC lakes are well known for at times prolific hatches of these insects and guys who imitate these hatches have there own unique way of fishing. Most people think it is mind numbing by I respect it for its effectiveness.
Here is an article about it:
Basically you fish an indicator on a long leader and adjust the depth of the fly as you fish until you find the right depth. The retrieve is a painfully slow hand twist(the mantra is you can always slow it down). It can sometimes take 20 minutes to retrieve a 60 ft cast.
Sounds fun right? Well it can be. Once you get dialed into the right depth and retrieve you can just nail fish after fish. I have seen guys talk 30 fish right next to me while I get skunked.
Needless to say I have had some luck with this technique but my patience is usually too short and I lose interested.
However if you can stare at an indicator for 10 hours a day it just may be the method for you.