"Isn't that the amount of time you can expect to get your limit of hatchery coho on the Vedder? "
Rod, believe it or not in 1986 ( EXPO, the year DFO decided to see how many coho local rivers could hold, or at least so it seemed) a four fish limit often took less time than that. If you retainined only selected fish, ie females for roe or larger males for the freezer, you could stretch your day to a few hours.
Sorry, the above has nothing to do with fishing in early July but I just had to add it. Remember the rainbows you will encounter are most likely hatchery steelhead juveniles that may possibly migrate out at a later date. I agree with the other posters that chinook fishing is better later in the month but there have been fish caught on opening day. Should be some good C&R char about as well.