Watershed delineation

[FaidessMwale]

I am new to GIS and ARCHYDRO in particular but knowledeable in hydrology. I am trying to delineate watersheds for my area. I am using Arcmap 9.3.1 and ArChydro 1.3 . The main river is an outlet of a lake; from the main lake, it flows into another small lake and emerges again. I also have an inland drainage lake within the same study area. I have followed the steps in ArcHydro and have read for over 2 weeks but nothing come close. I am a student and am in an institution where no one has knowledge ArcHYDRO. Your consideration to assist will be greatly appreciated.

I HAVE THESE SPECIFIC QUESTIONS.
1. Do the lakes need special attention?

2. I came across literature which said stream lines should first b checked for features such as braiding, brokebn streams, open waterbody boundary and dangling nodes as these may cause errors. How does one remove these in ArcMAP/ARCHYDRO? I found out that my streams has breaks.

3. These problems aside, when i follow all the steps, some of the delineated rivers are discontinous at some points; yet the actual rivers are continous, how do i deal with this?

4. When i use river gauges as batch points, i get watershed (if one comes at all) that are tiny or localised around that area. From my hydrology knowledge, a watershed covers an entire area upstream of a particular point.

5. Is there a way to merge watersheds? I want to have Hydrology Response Units that are similar to what is on paper map, which was manually delineated by my country in the 80's

Thank you

HELP PLEASE
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[MarkBoucher]

I found out that my streams has breaks.

Are you using the DEM Manipulation functions under Terrain Processing? If the streams are discontinuous, and you haven't burned in the streams, stream crossings such as bridges or culverts may cause a rise in the DEM that will "block" the flow direction and "stop" the stream.

Mark

[FaidessMwale]

Mark
Yes, am using the functions under terrain processing. I don't have bridges nor culverts in the study area. the streams were also burnt in. i dont know what is causing the delineated streams to be discontinous in certain places when the original stream is continous at that particular location.

Besides, I also noted when i do catchment delineation, i get catchments in most parts but in certain places the catchments dont get defined. what causes this?

Thank u

[MarkBoucher]

Have you run the fill sinks function?

I think the burn streams function only lowers the DEM where the streams are by a constant value. If there are localized low points in the stream, you would still have localized low points in the stream. The fill sinks function would fix this problem. Also, if the stream is not "wide enough" (at least 2 cells), you might have similar problems (though I'm not certain about this).

Mark

[LornaMurison]

It sounds like most of your problems (streams not joining up, tiny watersheds) are caused by errors in the DEM.
If you haven't looked at it already, get the document "Comprehensive Terrain Preprocessing Using Arc Hydro Tools".  Here is the link: http://support.esri.com/en/downloads/datamodel/detail/15 look under "Downloads-Case Studies" it is the last link.  That document explains what a lot of the terrain pre-processing tools do and under what circumstances you would want to use them.
As for the lake (by inland drainage do you mean lake with no outflow?) it depends on whether you are interested in it or not.  You could treat is as a sink (water flows in but not out) and give it it's own catchment within the larger one, or you could just fill it and incorporate it into the larger catchment.