Anchor Ice and Ice Dams.
Anchor ice creates a danger to fishermen and also adversely affects the fish population and the bug structure of a river. Anchor ice occurs when the bottom of the river freezes and makes fishing impossible. This only happens during a long cold snap, where the sun does not shine on the river and when the water flows are too low. The Frying Pan benefits from the outflow from Reudi which is a little warmer than the freestone water of the Roaring Fork near Aspen and therefore provided the flow is high enough, the water will not freeze. However, the narrowness of the Frying Pan Valley means that in the coldest months when the sun is lowest, if the flow is not maintained at adequate levels, the anchor ice will form.
The Roaring Fork River from Aspen to Basalt does get anchor ice and ice dams. At times these ice dams may break free sending a lot of water downstream. The Roaring Fork from Basalt down stream gets little anchor ice, for a several reasons. It has a lower elevation than Aspen. The Valley opens up below Basalt and gets more direct sun. Provided the flow is sufficient, the outflow from the Frying Pan is a little warmer.
However, the danger down-valley still arises where ice-dams break upstream sending water downstream. So the risk to fishermen arises principally at the time that the dams are breaking and the ice and water rushes towards the wading angler. If one is aware and careful of wading at these times the risks can be minimized. However if one is foolhardy, a sudden rush of water with ice may at the least fill your waders, or knock you over. So check with the local fishing shops and be mindful of the risks.
The Roaring Fork Conservancy has, in conjunction with Miller Ecological Consultants, been studying the phenomenon of anchor ice and its effect on fish habitat and food in the Frying Pan. Essentially the consultants have concluded what fishermen have always known, viz., that if the water flows from Reudi are too low in particularly cold weather and hence the water temperature is permitted to fall to the point where anchor ice forms, the consequence is a diminution in the feed biomass of the river. The anchor ice breaks free and scrapes the nymphs (stoneflies, mayflies and caddis) off the rocks and from the surface of the river bed. Miller Ecological Consultants monitored and recorded such a decrease in these invertebrates between 2002 and 2003 when the river flows were unseasonally low due to the drought and a significant amount of anchor ice formed. In contrast, those bugs such as the midges which live in the sediment and are more protected did not evince a population decline. The consequence was an imbalance in invertebrate structure which reduced diversity and reduced the total quantity of food available.
Although the study has not been conducted over a sufficient period, it is the view of the consultants that it can take up to 2 years for the biomass to regenerate to their original levels. (Reports can be view on the Roaring Fork Conservancy website - www.roaringfork.org). The obvious solution is for the water flows in the tail water to be increased during the coldest months. It was suggested that 60cfs might be a good target. However, given the competing interests in a drought year the necessity to balance various interests and claims means it is not an easy task. At present it would appear that there is a desire on the part of the Bureau of Reclamation to take the research into account and to attempt to regulate the water flows in the Frying Pan to permit greater flows in the colder months. More information is available on the Roaring Fork conservancy website.
On a positive note, when the anchor ice breaks free and drifts down stream it will scrape a lot of benthic macroinvertebrates off rocks and gravel surfaces making them drift and become vulnerable to the feeding fish. Consequently, at these times, fishing large stonefly nymphs will work successfully. However, as indicated, it will take a time for the biomass to regenerate and rebalance itself so these conditions are short-lived and are detrimental to the fishing for a year or two.