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I would to execute distributed hydrologic models centered on the State of Alaska. Because most of the basin headwaters begin within Canada, I am trying to find or build a single, harmonized GIS data for the entire hydrologic region. For my modeling, I like to have basins of approximately 100km2 area. I also use a single flow path to represent a “main stream” for each basin. Obviously, this single flow path is likely to be built from more than one actual “flowline” or NHN Flow_1 vectors. This is not a problem. What I would like to eliminate are all the tributary flowlines that are not on the main stream.

This probably needs some work, but captures ideas from an early discussion between Roland ( and Pete Steeves (

The NHD 12-digit WBD units are good for my use. I would like to create similarly sized units for the Canadian side of the border. I’m open to any of a number of approaches, including:

  • gathering analogous data from Yukon, BC folks. I would expect to need to do some aggregation/simplification of the Provinces’ data since I’m only working at a “regional” scale.

  • Have NASA DEM for northern Canada. Could prune Canadian streams to similar drainage density of “main stream” network on US side of border, burn those into the DEM (like the AGREE method), and derive catchments that way. For example, in figure below, I would find one “main stream” w/in each of the WBD units (to left of vertical international boundary), characterize that drainage density, then prune stuff on the right to similar density to define “main streams” within Canada, then use a DEM to find contributing areas to those features.

  • other ideas?

Pete Steeves: Roland, if  you need catchments then we need to add a DEM element to all this.  True?  We may have something on the Canadian side and along the border (border units).  The problem is areas completely in Alaska.  There is no NHDPlus or anything like it as far as I know.  We would  have to process data from scratch.

  • kinda/sorta on needing DEMs--I need contributing areas that are sized on the order of 12-digit WBDs. They don’t have to be “catchments” in the NHDPlus sense or even equal-area WBD-like HUs. I just need to be able to associate all land surface with a drainage line at not too coarse of a scale. So, for instance, the 12-digit WBDs are totally sufficient for my needs on the US side of the border. I don’t really care if they’re synthesized from DEMs or things I aggregate out of pre-existing linework. I will ultimately use a DEM to generate some parameters of the “contributing areas.” I expect that using Kris Verdin’s DEM would suffice there, but would happy to discuss alternate sources. Does that clarify? There’s definitely a ton of jargon to wade through, before we even get to the actual translation thing.

  • ok, I’m going to look into (ask NRCan) what the best way is to thin their data down to mainstems.  

    • I could see doing something like Larry Stanislawski’s thinning method--not that I really know how that works, but I think I could fake it til I make it.

    • Roland, I am communicating with NRCan on this.  I’ll get back on it soon.  What’s your time frame?

    • NRCan solution > FME Software.  Denis Boutin of NRCan is looking into this and will get back.  A question is, could the process work in the ArcGIS extension of FME (“Data Interoperability”)

  • Mainstems of arbitrary basins concepts:

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