Mike is entirely correct-even at this early date some insects have already
begun to emerge. I'm not sure why the willows, but all along many of our
streams winter stoneflies have been emerging.  The immature stages are
aquatic--frequently living in the water below the streambed-and the adults
of many of our common species are flightless. The adults emerge on sunny
days crawling up on bridge abutments, rocks, and vegetation along the
stream edges. These stoneflies can be abundant. Also emerging this time of
year are a number of the early season midges (small non-biting flies that
look like mosquitoes). Immature stages of midges are also aquatic and the
adults an abundant and frequent food source for organisms living along (and
in) steams.
A quick aside--now is the mating season for a number of our large
salamander species around here. Fishless streams and pools of water
frequently contain hundreds of these amphibians at night as they mate and
lay their eggs (even if their is some ice on the surface!)