Question About Water Features in DEM Maps

[ModelerMJ]

Hi All,

My route uses DEM terrain generated with TransDEM and data from the USGS Seamless Server. I have a few major rivers, and have noticed that they are very flat and shallow. After a bit of thought it occurred to me that the DEM data might be reflecting the height of the surface of the water, instead of the river bed, and that is why I'm seeing this effect. This would actually kind of make sense if I also assume my USGS data is actually SRTM data, since radar would probably not penetrate water surfaces.

I'm wondering if some of the more experienced folk here might have run into this before and be able to confirm my suspicions. If this is indeed the case, then of course the obvious thing to do is push down the river bed, i.e. with a road spline and the smooth spline tool, and then put my water at the original height given by the DEM data.

Thanks in advance.

Regards,
-Mike

 

[Dermmy]

You are right - it is the surface of the river that is shown in DEM data, not the river bed. On narrow streams etc lowering the river bed make sense but on larger rivers, lakes etc just setting the 'water' level a foot or two over the textured surface works well enough...

 

[wheelsonfire]

I ended up using a river spline on my DEM, and adjusted it manually to form a slow gradient. I initially tried to lower the river bed but by using the Trainz water level I ended up with river banks as deep as the Grand Canyon at the highest points. I would have preferred to use the Trainz water but I couldn't work out how to section the river without making it into a series of long skinny lakes.

 

[geophil]

Yes, it's the river surface. Here is a couple of examples for Bonneville Dam, Columbia River, OR/WA:

SRTM 1 arc sec:


NED 1 arc sec:


NED 1/3 arc sec:


NED 1/9 arc sec (as available):


NED 1/3 arc sec with US topo 1:24k overlay:

 

[steamboateng]

Indeed, it's the water surface. I do as Dermy mentioned; lower the river surface several feet to form the river banks. Google Earth pix help a lot to get the right effect, which may include sand/mud banks not easily noticable in the DEM..

 

[ModelerMJ]

Thank you to everyone for the replies, I appreciate the feedback and suggestions.

Regards,
-Mike