About 1.3% of all human caused gas emissions, made worse because it is primarily methane.
As for burning first, that only works for new dams, and you need to de extensive excavation to get at the roots as well- depending on the plant, upwards of half of its mass is below the surface.
Biomass decay isn't the only problem; the article also cites downstream algae blooms, which I suspect would be exacerbated by the occasional dredging necessary to keep dams from being blocked by silt.
A direct comparison to coal is tricky, because a lot depends on the type of coal burned and thr type of plant it burns in. Certainly coal isn't a clean source of energy, but it is clear that hydro isn't either.
Moreso, this can be minimized.
In the end, it's clear that hydro, long term, is indeed clean, moreso than a battery+solar solution would be.
Indeed, this is 1.3% going to 0.06%, despite the fact that hydro represents almost 15% of global electricty generation.
C02 produced by biomass varies with moisture content, but is generally on the level of coal per unit of energy. Moister wood produces slightly more, dry seasoned wood slightly less than coal.
how much are we talking about? does it work out to about the same as other fossil fuels, or order(s) of magnitude less? Also, is it possible to mitigate this by burning the vegetation prior to flooding the area?