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).
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