How to read a BC lake depth chart

Before anyone had sonar on a float tube, the Province of British Columbia went out in boats and sounded its lakes. Between the 1950s and the 1990s survey crews measured hundreds of lakes and drew each one as a contour map, a bathymetric chart. Those charts are still the best free picture of what lies under the surface of a BC lake, and every lake page on this site that has one links the Province's own file and says the year it was drawn.

Contour lines

Each line on the chart joins points of the same depth, like the contours on a topographic map turned upside down. The number on a line is its depth. Many older charts are in feet, not metres, so read the legend before you read the numbers; a 30 on a chart in feet is about 9 m. Where the lines crowd together the bottom falls away steeply. Where they spread apart it is a gentle, flat slope.

The shapes that hold fish

Shoals: wide flat areas, typically one to six metres deep, where the contours are far apart near shore. Light reaches the bottom, weed and insects grow, and trout come up to feed. On interior lakes the pale marl shoals show as bright turquoise from the shore.
The drop-off: the edge of a shoal, where the lines suddenly bunch. Fish cruise it, moving between the food on the flat and the safety of the deep. If you can fish only one line on a lake, fish this one.
Humps and sunken islands: a small closed ring of shallow water out in the middle. They are shoals nobody else has found, and they are easy to miss without the chart.
Points and saddles: a ridge of shallow water running out from a point, or a shallow neck between two basins. Fish travel along them.
Inlets and outlets: moving water, food and in spring cold water arriving. The chart marks the creek mouths.

What else is printed on the sheet

Most Province charts also give the surface area, the maximum and often the mean depth, the elevation and the survey date. Mean depth is worth a look. Two lakes with the same maximum depth can be one deep hole in a shallow basin or a deep bowl all the way across, and the shallow basin warms faster, weeds up more and is more exposed to winterkill. The inlets and outlets are marked too.

What a chart cannot tell you

A chart is a picture of the basin on the day it was surveyed. A reservoir that is drawn down in late summer may be metres lower than the chart, and a lake raised by a beaver dam may be higher. Weed beds come and go and are not drawn. The chart does not show where the fish are today, only where the structure is. Use it to choose where to start, then let the water teach you the rest.

Which lakes have one

Each chart is matched to its lake by the Province's waterbody id and by the name printed on the sheet, because an id can carry a sheet drawn for a neighbouring lake and linking that would send you to the wrong water. The counts below are the lakes where the match held. A few of the biggest lakes, never charted whole by the Province, link the federal Canadian Hydrographic Service nautical chart instead.

Lakes with the Province's surveyed depth chart linked
Region	Lakes in the atlas	With a Province depth chart
Vancouver Island	193	121
Lower Mainland	133	81
Thompson-Nicola	341	120
Kootenays	162	76
Cariboo	328	218
Skeena	183	94
Omineca	235	146
Peace	54	38
Okanagan	213	83

977 lakes in all. Each chart is matched to its lake by the waterbody id and by the name printed on the sheet, and a sheet drawn for a different lake is left off.

Putting it together

Open the lake's page, follow the Province chart link, and find the shoals and the drop-off before you leave home.
In spring, fish the shoals. In summer heat, fish the drop-off and the deep side of it. In fall, the shoals again.
Check the wind on the lake's forecast. A big shoal on the windward shore is a long, rough row on a gusty day.
If a chart is clearly wrong, or matched to the wrong lake, tell us through Report Data Fix in the app. Readers' reports are how the atlas gets corrected.

Open the map: 1,842 BC lakes, each with a 7-day forecast and a score