It has been cold (as my recent postings about ice on the Lake and creeks attest). Weather forecasts suggest that the air temperature is about to rise and is unlikely to plummet as low again this winter. This may be the last time this season to visit the freezing-weather travails of a dipper caught between the twin scourges of surface and anchor ice.
Surface ice and anchor ice form in different portions of a creek. Where the flow is gentle, such as near the creek mouth, ice will cover the water’s surface, but anchor ice will not form. Where the flow is turbulent, such as where the creek is steep, anchor ice covers the creek bottom, but surface ice is inhibited from bridging across the water.
What is a poor dipper to do? It normally finds its food anywhere along the length of the creek bottom, whether the flow is smooth or turbulent. It is now blocked by twin barriers.
Curiously, at present temperatures, Kokanee Creek (where I make these observations) has a Goldilocks zone: one that is not so turbulent that anchor ice forms, and not so smooth that surface ice bridges across. That is where dippers go—where the turbulence and ice are just right. Would the dipper’s Goldilocks zone vanish with another 10C drop in temperature? We are unlikely to find out this winter.
Do other local creeks have a Goldilocks zone for dippers? I have no idea. But, the existence of such a zone may play a role in the long-term suitability of a creek for dippers.
Where the flow is turbulent, anchor ice covers the creek bottom, but surface ice has difficulty bridging the stream. Dipper foraging is out of the question here. 
This dipper has been foraging in the waters of Kokanee Creek’s Goldilocks zone: the flow is too calm for anchor ice, but too turbulent for ice to bridge the surface. It is just right for dippers.
After posting the material, above, I realized that I should show a picture near the creek mouth where the flow is so gentle that surface ice covers everything. So, I visited the creek again and took this panorama. The marks are from boots and skates. They certainly show that, under the dusting of snow, there is a solid cover of ice. No dippers can hunt here. Yet, when I walked past the Goldilocks zone again, three dippers were foraging.