1. The fish you find are more likely to be winter survivors rather than migrants from downstream. I say this mainly because fish don't migrate as far as most people think they do, and I'm speaking here of resident populations. As long as there is available water with sufficient oxygen, it is likely that they will survive; I'm assuming an adequate food supply. What you're likely to find in these situations is that the fish tend to congregate in the deeper pools during the winter and then move out to the riffles.

2. As I said above, as long as the pool is large enough to have free water that is adequately oxygenated, the fish will survive; surface ice is rarely a problem in streams. The fish do need some food, although metabolic functions are at low ebb when the water is near freezing. There are a couple of conditions that are detrimental to the fish. You mention one of them and that is when the water freezes solidly; obviously, this will kill the trout but is fairly rare where adequate flows prevail. The other condition occurs when the water reaches super cooled conditions (clear, cold nights) while it is still flowing and the surface is unfrozen. At this time, ice crystals (called "frazil ice") occur as slush in the water, or ice forms on the bottom (called "anchor ice"). Anchor ice forms initially in riffles, but can spread to larger areas if low temperatures prevail. Break-up of anchor ice scours the bottom, resulting in a loss of immature insects, abrasion of the periphyton community, and, in some cases, transport of embedded stones and gravel.

3. This one is harder to answer because it could vary with several things, especially the severity of the winter and adequacy of the water supply. Given that there is adequate flow throughout the winter, the chances of the stream freezing solidly beneath the snow pack probably depends on how cold the air temperatures are and how long they last. Prolonged sub-zero temperatures are more likely to result in a greater portion of the water freezing; conversely, warmer air temperatures will probably allow the water to remain flowing. Also, as you imply, the thickness of the snow pack may affect how much of the water column freezes. ~ Bert

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.