Why would channelizing a river reduce any nearby wetlands? Channelization is when you straighten a river to make a shorter path from A to B. This makes the water flow faster, since the path is shorter but the volumetric flow rate of water must be the same (because it's receiving the same rate of water from the watershed).
However, that wikipedia article also says,

Channelization has several predictable and negative effects. One of them is loss of wetlands.

No citation and no explanation. I don't get it. Why would nearby wetlands be lost from this process? How could that happen?
Channelizing a river keeps it from overflowing its banks as often, which prevents flooding.  However, wetlands need a certain degree of said flooding to function as a biome.