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I'm having a rather horrible issue where my rivers decide to be straight lines instead of following the DEM I am using. Can someone explain why this is happening, and what I can do to solve this problem aside from manually routing rivers?

UPDATE: Using a Low Pass Filter allowed expected channels to be created in the lower areas of the map, but prevented them from generating at higher elevations at all.

UPDATE 2: Due to GRASS's r.watersheds crashing QGIS when I try to use it, I am using SAGA's Channels tools.

Solution: r.watersheds can be run outside of QGIS using GRASS as a standalone program. This works on windows in spite of r.watershed hanging infinitely within QGIS.

The problem I am having

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    Sometimes integer DEMs give line generation a hard time. If you haven't tried using a low-pass filter to convert the DEM to float, you should probably do that first.
    – Vince
    Jul 11, 2022 at 4:35
  • @Vince I just gave that a shot. It fixed the lower elevation areas but prevented higher elevations from generating channels at all. Is there any way I could hybridize two DEMs by a given region so I could use some of column A and some of column B to get the channels done everywhere? Jul 11, 2022 at 5:29
  • You could certainly try, though I can't imagine why it would be necessary. What coefficients did you use for the filter? I've only tried 0.001x8+0.992.
    – Vince
    Jul 11, 2022 at 8:44
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    Is the information in this of any help? It looks like you should try some different minimum slopes. gis.stackexchange.com/questions/374056/… Whitebox tools in QGIS has stream network analysis tools IF you have the plugin already installed. The plugin's repository is currently unavailable. GRASS is working for me in 3.26. What version of QGIS and SAGA are you using?
    – John
    Jul 11, 2022 at 23:13
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    I have been using GRASS in QGIS 3.26 on some big data sets with up to 30 hours runtime. I recommend this simple guide: giscrack.com/automatically-delineate-a-watershed-in-qgis Be aware of memory issues- e.g. default memory use needs to be increased on my system and I need to enable disk swapping. Also the program writes many files to a temporary directory, which can cause issues. Finally this free acces recent article has a good discussion of DEM processing: doi.org/10.1080/19475683.2019.1604571 Jul 12, 2022 at 0:46

2 Answers 2

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What did you use to create the streams? It seems you have some low lying areas and certain hydrology algorithms do a "fill depressions" in advance to allow routing the streams. The GRASS GIS module r.watershed, for example, avoids this, using an MFD algorithm that is able to search for the "best" or "prettiest" route through low/flat areas.

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  • I can't use r.watershed, it crashes QGIS for me every time I try. I looked into this and apparently, a lot of GRASS plugins are just broken for Windows right now. I used SAGA's Channels and Basins for this. Jul 11, 2022 at 13:33
  • There is always the option to run GRASS on its own, without QGIS.
    – Micha
    Jul 12, 2022 at 6:10
  • I'm trying to do that at the moment. I'm new to this. I'm not sure how exactly you do that, and cannot find a good tutorial. Jul 12, 2022 at 6:33
  • Maybe some tips here
    – Micha
    Jul 12, 2022 at 11:38
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Which algorithm did you use for DEM preprocessing? I've had that issue sometimes when working with "medium-high" resolution DEM. Anyway, the problem is quiet common when analyzing flat areas. Try with SAGA the different algorithms within Terrain analysis --> preprocessing and create channels with each of them in order to see which one fits better with the actual drainage pattern. Once I had that problem and the only way to avoid the straight streams was using "sink removal" instead of "fillsinks" and then calculating channel network. Hope can be useful

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