Shallow waters, a raft and the web spell. What happens?

Tonight a band of lizardfolk was towing a large raft (some 30-feet square raft) over shallow swamp waters. Some 4~5 feet deep. Since the raft had some several key townsfolk tied up on it and was on its way to be sacrificed to some hideous demi-human deity, the wizard decided to cast web below the raft.

His rationale was that the web would anchor between the raft and the swamp floor, trapping some of the lizardfolk but also stopping the raft from moving.

He was underwater, had water breathing on, and could see underneath the raft, so targeting was not an issue.

Would the web anchor (prevent movement of) the raft?

I would say yes, because the Web spell can be layered over the bottom of the raft, and the web spell says that:

Webs layered over a flat surface have a depth of 5 feet.

Since this is, or more than, the depth of the river, the web could be considered anchored to both points, as it would be the exactly the same if you cast web on the floor or the raft.

The creatures pulling it would be able to make a dex save to not get entangled, but if they do get trapped then they must make a strength check to break free. You could consider the raft trapped by this and therefore unable to move, but any free creature would be able to make that strength check for the raft to break it free.

The web spell can't hold the raft

The web spell says:

If the webs aren't anchored between two solid masses (such as walls or trees) or layered across a floor, wall, or ceiling, the conjured web collapses on itself, and the spell ends at the start of your next turn. Webs layered over a flat surface have a depth of 5 feet.

It seems like the underside of the raft is being used as a solid mass, and the types of masses the spell expects are walls or trees -- ie, sturdy and unlikely to collapse.

This implies the structural integrity of the web relies on the surfaces it's bound to, not the other way around. If the structures can't hold the web in place, the web collapses and the spell ends.

Moreover, there is no guarantee that the bottom of the river is solid in the way a tree or wall is solid. It could be made of mud or silt, offering no anchor points for the spell.

Alternatives

This question looks at ways to increase weight of objects for the purpose of anchoring them down.

• Enlarge/Reduce spell multiplies the target's weight by 8

• Flesh to Stone spell multiplies the target's weight by 10

• Immovable rod item cannot be moved unless the mover succeeds a DC 30 Str check

• Comments are not for extended discussion; this conversation has been moved to chat. – SevenSidedDie Jun 17 '17 at 17:52