My first big river, in 2014, was the Klamath. At first, I tried looking for it using the OSM search box (Nominatim). All I found was a mess of missing river parts, and when I looked closer, I found poorly imported NHD, very old and wrong riverbanks, incorrect tagging, etc. I spend a few days to fix it up and produce the Klamath River waterway relation:

Since then, I’ve done similar work for other waterways. Sometimes the relation exists but is incomplete, other times I create it; either way, it can take days of work to finish. Here are some in the USA:

• Housatonic River
• Black River (NC)
• Denver’s High Line Canal
• Sacramento River
• Iroquois River
• Rogue River

Recent work in Illinois:

• Rock River
• Big Bureau Creek

Outside the USA:

• Río Sinaloa (Mexico)
• Mtamvuna River (South Africa)
• Demerara River (Guyana)
• Water of Leith (Scotland)