Depth, and where the trout are

Two facts sit at the top of every researched lake's sheet: how big the lake is, and how deep it goes. Size tells you what the wind will do. Depth tells you where the fish will be, and it changes with the season.

Spring: shallow and hungry

After ice-off the whole lake is close to the same cold temperature and the shallows warm first. That is where the insects start and that is where the trout go. A shallow lake, or the shallow end of a deep one, fishes best in the weeks after the ice leaves. Depth matters least right now.

Summer: the thermocline

By July a deep lake has sorted itself into layers. The top few metres are warm and low in oxygen, the bottom is cold and dark, and between them sits a band where the temperature drops fast. Trout hold in and just above that band, because it is the coolest water that still carries enough oxygen. On a lake 30 m deep that band may sit 6 to 10 m down. On a lake 6 m deep there is no band at all, and a hot week can leave the fish nowhere comfortable to go. That is why a deep lake keeps fishing through August when a shallow one goes quiet, and why the maximum depth on the sheet is worth a look before you drive.

Fall: the turnover

When the surface cools to match the deep water the layers collapse and the lake mixes top to bottom. For a week or two the water can look dirty and the fishing can be slow. Then the whole lake is cool and oxygenated again, the trout spread out, and the shallows fish well until freeze-up. Shallow lakes turn over earlier and more gently than deep ones.

Where the depth on the sheet comes from

Between the 1950s and the 1990s the Province sounded hundreds of BC lakes and drew each one as a contour map. Where one of those charts matches a lake, by its waterbody id and by the name printed on the sheet, the lake's page links the Province's own file and says the year it was surveyed. Where the chart could be read and its outline matched the lake's own area, the maximum depth on the sheet now comes from that chart, and where it disagreed with an older record the chart won. The lakes imported from the Province's Freshwater Atlas carry no depth unless a chart gives them one, and the sheet says unknown rather than inventing a number. The guide to reading a depth chart walks through what to look for once you have one open.

How to use it

• In spring, do not let a shallow number put you off. It is the best time for that water.
• In summer heat, choose the deeper lake, and fish the band where the temperature drops rather than the surface.
• After the first cold snap of fall, give a deep lake a week to settle, then fish the edges.
• Check the seven-day forecast on the lake's page for the wind, and open it in the app for the score.