It's the middle of winter, and most Yukon lakes and rivers are covered with thick layers of ice. Nevertheless, dozens of monitoring stations around the territory are still measuring their water levels and flow rates.

Winter, when free-flowing water is at its lowest, is the most difficult and often the most important time for monitoring the flow in many streams, says Russ Gregory, Acting Head of Yukon Operations for the Water Survey section of Environment Canada.

"Our instruments are not designed for incredibly small flows, yet that's one of the most critical things we have to determine."

At high water levels, a small amount of pollution might be diluted by the stream to the point where it poses little hazard. But in winter, with the flow in many streams reduced to a trickle, a small amount of pollution could be devastating.

The presence of flow in winter also determines whether a stream can support fish like salmon, adds Gregory. Since salmon-bearing streams receive a high level of legislative protection, information about winter flow is critical to industry and government.

"The smallest trickle will mean the difference between a salmon-bearing stream and a stream that doesn't bear salmon. So even though it's very, very difficult to measure that low flow, it's important to know that it's not zero."

The Yukon office of Water Survey maintains 67 monitoring stations. They are all automated, and about a quarter of them transmit data to a communications satellite several times a day. The remainder store the data on site.

"We go as far south as Dease Lake and as far north as the North Slope," Gregory says. "We also go into Alaska, along the border. We have stations which are operated jointly with the U.S. Geological Survey."

"Typically the station looks like an upscale outhouse, a steel outhouse," says Gregory. "There will be a solar panel on the side, an antenna on the roof, and that's all you see on the outside. From the structure there will be one line going into the water, which is the sensing outlet to the river."

Water Survey staff visit all the sites at least six times a year to physically measure and take samples by hand and to collect stored data from stations that do not have transmitters. The primary measurements are stage (water level) and discharge (flow rate). The relationship between the two factors provides valuable information about the state of the stream.

On-site visits are vital. "You have to be on top of the processes or you can't calibrate that relationship. Less than six times a year and you're grasping at straws," says Gregory.

"The physical characteristics of the basin and channel must be observed and recorded regularly by the technologist, and the recording equipment requires regular maintenance."

In winter, the on-site visits are even more important because of the effects of ice on the streams and the instruments.

"Ice is variable and it changes the physical characteristics of the channel throughout the winter," says Gregory. "Often it's frozen to bottom. Also, in a lot of the channels that you see frozen over, up to 80 or 90 percent of the channel underneath the ice is packed-solid slush. It'll pack in there and destroy any submerged equipment you have or block it off."

The solution is to take samples through the ice at intervals along the stream, and interpolate what's happening in the stream between the site visits. It's difficult, expensive, and not altogether satisfactory, but it's the best method available so far, says Gregory.

"It's a real unknown area, winter flows. It's something that needs a lot of study."

For more information about monitoring Yukon lakes and streams, contact the Water Survey section of Environment Canada, Whitehorse, 668-2161.