Reading a lake depth map

A contour map shows what the bottom of a lake looks like, which is the part you cannot see. For many Texas reservoirs the Texas Water Development Board (TWDB) has surveyed the lake and published one. This guide covers where those maps come from, what the numbers on the lines mean, how to turn a line into a depth for today's lake level, and the shapes that matter to an angler.

Where the map comes from

TWDB says its Hydrographic Survey Program was created in 1991 under Texas law to survey the storage capacity of reservoirs, and that it has done about 200 surveys on 116 reservoirs since 1992. A volumetric survey produces updated elevation-area-capacity tables and a bathymetric contour map, using acoustic data. TWDB's Completed Surveys & Data page lists each survey with its map. Each map is labeled with the survey month and year (the Inks Lake map says August 2021) and a contour interval in its title, such as a 5-foot contour map. The date matters: a lake's bottom changes as silt settles, and TWDB's separate sedimentation surveys measure that. TWDB's own wording on the Inks Lake map is that it makes no representations and assumes no liability, so treat a map as a guide to the bottom and not as a chart to navigate by.

The numbers are elevations, not depths

This is the part that trips people up. The number on a contour line is an elevation: feet above mean sea level (or another named reference), the same kind of number as the conservation pool. TWDB's Inks Lake map marks the conservation pool elevation at 888.0 feet MSL and the top of the dam at 922.0 feet, and labels its underwater lines from 830 to 885 feet in 5-foot steps. At conservation pool the water over the 830-foot line is 888 - 830 = 58 feet deep. When the lake is lower, there is less water over every line; when it is higher, more. So to find a depth, take the water surface elevation and subtract the line's number. The lake level drawing uses an example lake with a conservation pool of 500 feet so you can try it. TexCast's lake levels article explains the conservation pool and how far from full a lake is.

An interactive drawing goes here in the app.

Check the reference. TWDB maps do not all use the same one: the Inks Lake map says feet above mean sea level, and the Lake Houston map says feet NAVD88. Use a lake level that is stated in the same reference as the map, and take the conservation pool number from the map itself.

Line spacing is slope

On a map where every line is the same number of feet apart in height, the distance between lines on the ground tells you how fast the bottom falls. Lines bunched together mean the bottom drops quickly; lines spread far apart mean it drops slowly. You can turn that into a number: the height between two lines divided by the distance between them. The drawing does it for 2-foot and 5-foot lines, which are the intervals TWDB shows in its map titles. Nobody publishes a number at which a drop counts as steep, so TexCast gives you the number and leaves the call to you.

An interactive drawing goes here in the app.

Shapes to look for

A few shapes show up on almost every lake. A point is the bulge of land going out into the water. A creek channel is the old creek bed, drawn as lines that dip into a V or U toward the creek. A hump is a set of closed rings with the smallest ring on top. A flat is a long run of bottom with few lines. The shape tells you where fish may be: TPWD says the best places to look for spawning crappie are shallow flats next to creek channels, and its crappie brochure lists submerged creek channels, submerged islands and cover among the places to look; see crappie fishing in Texas and bass fishing in Texas. Try each shape, and the lake at full pool and 10 feet low, to see which tops go dry.

An interactive drawing goes here in the app.

Using the map on the water

The lake in the drawings is an example, not a real map. This is a summary of how TWDB maps are laid out; the map itself, its date and its reference decide what its numbers mean. TexCast shows readings from public sources and is not a navigation or warning service.

Related

For more reading, we suggest these articles

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