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I need to summarize downstream information associated with each arc in an NHD dataset. I have a table with the downstream ID for stream segment but am not sure how to get all of the downstream IDs. Any ideas?

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Are you using NHDPlus? If you can deal with 1:100k data (instead of the high-res 1:24k NHD), the Value Added Attribute tables of NHDPlus allow for extremely rapid downstream navigation. –  dmahr May 14 '13 at 22:37
    
Have you seen RivEX (www.rivex.co.uk) it can generate an adjacency table which you use to traverse upstream or downstream? –  Hornbydd May 15 '13 at 10:28
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