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?
Length of Rivers determines rate of flow. When you straighten a river you increase it's flow rate and make it MORE prone to flooding downstream. Cities try to curb this by turning their rivers into huge drainage swaths (LA river) but ultimately it's pointless. Winding rivers desposit sediments and silt (and nutrients) on lands, those feed wetlands necessary soil amendments they otherwise could not obtain
What is the purpose of channeling a river, to make it move water more quickly. Moving water more quickly is also called draining water more effectively, If it draining water more quickly the land around it gets drier.