When I first took my boat on a river, it was a scary experience. Whereas canals are shallow and (mostly) have no flow – so even an engine failure isn’t really a disaster as you can always wade to shore if needed – rivers have a current and can be deep; they have weirs with rows of buoys stopping you and your boat from going over the rocky incline, and just a little rainfall in the prior few days could make all the difference to the feel of the water. It was a whole different ballgame.
For that reason, a little more care and preparation is advised before taking your boat on a river (and for tidal river stretches, further preparation still). It’s worth having a suitable anchor with appropriate length of chain, tied to the bow of your boat such that in an emergency it can be thrown overboard to stop you drifting. You and anyone else aboard should probably wear a life jacket. Without question you should do at least basic engine checks before you set off (ensuring the fuel is clean, the engine running well, the oil level topped up etc)
Locks on some rivers (eg the Severn) will be manned and require a phone call in advance of your arrival. Others will be self-operated but may have hydraulic systems to work the gates and paddles, requiring you merely to press some buttons rather than work them with a windlass. Every river seems to be different so do ensure you read up in advance of travelling on them.
You’ll also find rivers can be prone to silting up on bends – which you may end up stuck on if you take the corner too tightly.
There are some river sections that rise and fall with the tide (eg the Trent once you go above Cromwell lock, up to Torksey) and you’ll need to plan your excursion along here in conjunction with the lock-keepers at either end. A VHF radio can be handy (for which you need a formal qualification to use legally) and red/green navigation lights may be required by the river authority, if you plan of travelling at night.