D&D General Setting a campaign on a river

[Yora]

As an alternative to rowers, in places where sails can't be used, that's a pretty cool idea.

 

[GuyBoy]

Giant Water Beetles are pretty savage.
Also giant pond skaters moving so quickly across the surface.
Dragonflies hunt on river surfaces.

There’s also the Dracula scenario with some form of deadly cargo aboard and “disappearances.”

 

[MattW]

How about a giant version of the Diving Bell Spider?

This giant spider could be both useful and terrifying for those who explore the underwater ruins.

 

[Eltab]

Utricularia (sp?) - a carnivorous plant that lives in shallow water. IRL, the suction-based trap can be triggered by bugs, newborn fish fry, and such that tickle a sensor hair.
Scaled-up version might grab your foot and cost you movement / a bonus action rather than sucking you in whole.

Quicksand near river bank

Mud bog under shallow water so it "suddenly gets deeper" and takes extra movement to get through and might even behave like quicksand with a layer of drowning on top.

Murky water conceals a drop-off. Overconfident characters in Heavy Armor will not appreciate "discovering" 7-foot depth. And an intelligent foe might do some underwater digging beforehand.

 

[GuyBoy]

Floods. Especially flash floods.

 

[Yora]

I love dragonflies. Totally gonna use those. I believe that also means mosquitoes in swampy forest and wet moors.
Fighting in soft mud also is an idea I really love. Might actually come up quite often when PCs try to make a dash for their boats to escape land monsters.
Water plants that can latch up to swimmers and whole boats are also a cool idea. Might not be a big problem by itself, but could have a big impact on some fights.

So anyway, I made a nicer map.

The main structure is from the Mississippi, scaled to half its length, with some altered branches.

 

[Rabulias]

Also, what if the river ever freezes over? That could provide a nice environmental obstacle, and/or provide an opportunity for iceriggers akin to Dragonlance's, as seen in the cover to DL6: Dragons of Ice:

 

[Hussar]

If you saunter over to the Canonfire! site, there's an entire 20 level 5e campaign set on the rivers of Greyhawk.

Here's the introduction: The Great Flanaess River Adventure Introduction

 

[havard]

Cool idea.
In river based adventures I have DMed and played in, we have often ruled that trying to steer a river boat at night is too dangerous, so they always have to dock for the night. In 5E, I might also rule that you need to set up camp on the banks in order to get a Long Rest.

This allows for various combat and non-combat encounters on land and you arent always stuck on the boat which can get a bit repetitive. I did see that the OP has plans for other ways to get the PCs off the boat though.

-Havard

 

[Yora]

At the end of the day, the river is a means to get to places

If you saunter over to the Canonfire! site, there's an entire 20 level 5e campaign set on the rivers of Greyhawk.

Here's the introduction: The Great Flanaess River Adventure Introduction

That link is dead. But this one isn't.