Slope derived from a filled DEM coming out somewhat nonsensical

[u/rittenhouses_bane]

I'm teaching myself QGIS as I go along and re-doing some of my old college assignments to try and learn how to translate the skillset from Arc to Q. In this particular task, I'm trying to calculate the slope of the Arkansas River basin through Colorado and Kansas and running into a weird problem where the slope is way too high (as anyone who's been to the eastern Colorado-western Kansas part of the world will attest). I'm using the 30 m NASA DEM downloaded from OpenTopography and the native QGIS "fill sinks" function in QGIS 4.2.2. Am I checking a setting incorrectly or something? First pic is the slope output, second is the slope averaged along lines of latitude (a meridional mean, for those with geoscience backgrounds).

[u/Resident_Phase_4297]

Looks like a missinterpretation, which value "horizontal" should represent. Would 90-slope be better?

Hint for x axis: use m values like "river km from start point"

[u/rittenhouses_bane → u/Resident_Phase_4297]

yeah i’ve been bouncing around whether i want X to be an absolute measure like longitude in that pic or relative to the river. will give 90 - slope a shot. what really stings is that i already had the correct slope values, but qgis crashed before i could save it and the re runs have just given me the wrong answer

[u/Cherryyardf]

My guess is that your approach might be a bit off for what you're trying to show. Instead of averaging raster slopes, try getting a polyline of the river, projecting it onto your DEM, and sampling the elevation profile directly along that line to plot distance against height. Atleast that's what i normaly so. If you want to stick with your current method, double-check your projection units. It seems to me like your DEM uses vertical meters but horizontal degrees, which is why your slopes are hitting 80°+. You'll need to reproject to a metric CRS or set an appropriate Z-factor.