Seems like (more anecdotal wisdom from me, worth every penny you paid) the first few rains of the season (unless they are legit gully-washers) tend to get soaked up in large part by the summer-parched earth. Once the soil saturation increases, the rain starts washing down the valleys more quickly, which is what blows out the creeks, then the rivers. Hopefully, we'll reach that saturation point toward the end of next week (or, maybe that's what NOAA and the USGS are thinking). If the forecast holds (it won't ;-)), the light rain over a long period could mean healthy flows that are still green to clean (at least in the Lower basin).