The vast majority is plant matter. It wastes a lot of energy to chase prey. Plants don't move much. Last I read it was on the order of 80% plants, with a lot of insects being most of the meat.
[1] https://explore.org/livecams/brown-bears/brown-bear-salmon-c...
Edit to add:
Here's an article [2] from 2018, How many salmon will a bear eat?
Excerpt: Adult male bears on Kodiak ate the most salmon on average, consuming an incredible 6,146 pounds (2,788 kg) per bear per year! Adult females ate 3,007 pounds (1,364 kg). Salmon consumption varied among subadult bears, independent juvenile bears between 2.5 and 5.5 years old. Subadult females ate 1,248 pounds (566 kg) while subadult males ate 1,305 pounds (592 kg) of salmon per bear per year.
[2] https://medium.com/@exploreorg/how-many-salmon-will-a-bear-e...
It's not just standing and snatching fish out of the stream, though. Most bears have to chase the fish and try to catch them, often failing.
They get much larger and much fatter than the inland bears as a result.
Approximately 45 ± 22% (equation image ± SD) of the assimilated nitrogen consumed by male grizzly bears, 38 ± 20% by female grizzly bears, and 23 ± 7% by male and female black bears came from animal matter.
Diet varies a lot by region. Figure 7 here: <https://www.researchgate.net/profile/Bruce-Mclellan/publicat...>
Shows that for female grizzlies without salmon in their diets, % meat seems to vary from around 0% to 60% based on population.