understanding contours

[GoneFishingLTN]

I'm a little lost on how to truly understand contours. I know how to read them but I don't understand by lookin at the contours its probably a hard bottom. Any suggestions?

[Chris Catignani]

Contours will only tell you elevation...not content.

[padon]

yeah its hard to tell bottom composition by looking at contours.now sometimes high spots will be hard because the water hasnt been able to wear the bottom away there. but thats just a guess. the contours will give you a place to start. something to look at. but youstill have to go over there idle around and look.

[Tatsu Dave]

My garmin tells me hard from soft bottoms, as already posted the contours just show how steep or gradual the bottom comes up. A good depth finder does it all and gives you a pretty good picture of what is under your boat.

[Bankc]

At best, gradual changes in the contours often indicate a softer bottom, as it's hard for soft mud or sand to produce a steep cliff without eroding away.  And while gradual slopes might indicate a soft bottom, large flats surrounded by steep elevation changes might indicate shale or something like that.  Rough bottoms with lots of quick, extreme elevation changes might be hard rocks.  But all of that is just speculation.  There are many reasons why the opposite could also be true, like the effects of currents and the shape of the water body around that area.  So you can't really do more than a rough guess with just a contour map.  

 

Barring sonar, the best way to determine bottom composition is with a heavy jig or Carolina rig or something.  Just drag is across the bottom and get a feel for the resistance.  Sometimes you can feel the difference between mud, sand, riprap, large rocks, or shale.  Or maybe drop and anchor and see what it pulls up or how it digs in.  And if you're near the bank, sometimes it's easy to guess what's below you by what runs up the bank.  

[J Francho]

You can only guess, until you actually ride over it with your sonar on it.

[GaryH]

5 hours ago, GoneFishingLTN said:

I'm a little lost on how to truly understand contours. I know how to read them but I don't understand by lookin at the contours its probably a hard bottom. Any suggestions?

Are you looking at the contours on a map or on a graph?

[WRB]

To understand contour lines you should start with a topographical map of the area you live or fishing.

Most topo maps are in 5’ elevation increments, each line represents a 5’ change.

You can visually see the terrain to interpret the meaning of the map lines without water. Gentle sloping terrain the lines are further apart, steeper the line are closer together.

By being able to visualize the terrain by looking at the topo map is important to be able to visualize the terrain underwater. The surrounding terrain doesn’t change covered  water. 

Soil types like soft mud, sand, gravel, rocks, stream or river beds are very similar to your local area. If you see a mud bank above water that bank more then likely extends underwater, same with sand, gravel and rocks. 

Your sonar returns are brighter or different color on harder surfaces like gravel or rock and less sharp colors on softer soils. 

Vegetation doesn’t grow without softer soil. Clay is a form of fine sand, gravel and rocks don’t support aquatic vegetation like reeds or weed beds. You can tell a lot by knowing what is growing above the water to determine what is underwater.

Soil transitions are just as important as elevation changes, both form break lines.

Hope this helps.

Tom

[Catt]

5 hours ago, WRB said:

You can tell a lot by knowing what is growing above the water to determine what is underwater.

 

Can't tell y'all how many hundreds of hours I spent riding around Toledo Bend just looking at the shoreline & graphing. I also spent countless hours riding back roads looking at the topography.

 

8 hours ago, J Francho said:

You can only guess, until you actually ride over it with your sonar on it.

 

[Hawkeye21]

You can guess pretty closely what the bottom is like from looking at the contour.  Flats and gradual slopes are usually going to be a softer bottom, like mud and sand.  Sharper drop offs are almost always going to be rocky, usually bluff rock.