New editor here. I’m looking at the Ottawa River in iD and I’m perplexed by the river geometry. Is what I’m seeing an error, or an issue with iD?
Looks like a rendering issue with multipolygons because not all of the ways referenced by the multipolygon have been downloaded.
I see the same issue in JOSM as well. Right up until I download this way here (just East of Lower Duck Island).
Try zooming in on that section of the map so that the way is downloaded and it should fix the rendering issue.
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Hello MitMusik and welcome. 
I think what you’re seeing here is just a rendering artefact when you’re using iD. The water area of the Ottawa River is a multipolygon relation composed of about two dozen smaller line segments (Relation: 13034321 | OpenStreetMap). Your browser hadn’t loaded all of the member line segments, so it shows a screwy open-ended area.
If you select one of the members of the relation, and open the relation itself, your screen will look like this:
If you scroll down the sidebar, you’ll see all the members of the relation; in my case, only a few were downloaded by iD:
You can manually push the iD editor to download them by clicking the ‘download’ button on the top right corner of each “not downloaded” way. After doing so, it’ll look like this:
Ta-da! 
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That makes a lot of sense. Thanks for your answers!
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