The reason I like to fish these streams in the first part of the season is simple. They're fed mostly by run-off & ground water, so they heat up much sooner in the year than the medium & larger size freestone streams. It's at this early part of the year when the fishing is best in these smaller streams.
Any hatches that occur will most likely be small in numbers & duration, but are sure to bring the fish up to the top. However, hatches aren't the most important thing on these streams. You can pick up fish all day long with just about any reasonable fly so long as you plan out every move you make. After fishing this way all day, you'll be tired both physically & mentally but it's that good kind of tiredness - the type that let's you know you spent a day well in a beautiful place doing what you love!