Smooth river beds make for smooth, faster flows; rugged or weedy bottoms create turbulence and slow things down. Here's a fairly lengthy list of sample values (gleaned from https://www.engineeringtoolbox.com/mannings-roughness-d_799.html), but to be honest, you could probably get away with just using 0.035 as a default for most rivers. Follow that link if you want to work out flow rates for things like sewers...there's a good range of materials there.
|Earth channel - clean||0.022|
|Earth channel - gravelly||0.025|
|Earth channel - weedy||0.03|
|Earth channel - stony, cobbles||0.035|
|Floodplains - pasture, farmland||0.035|
|Floodplains - light brush||0.05|
|Floodplains - heavy brush||0.075|
|Floodplains - trees||0.15|
|Natural streams - clean and straight||0.03|
|Natural streams - major rivers||0.035|
|Natural streams - sluggish with deep pools||0.04|
|Natural channels, very poor condition||0.06|
The last two factors we need to figure out are width and depth, and that is going to be a fairly lengthy, math-heavy post. But first, next time, I'll go over some different types of rivers and make at least a general link between width and depth, before taking that plunge (I couldn't resist).