This isn't worth its own post, so I'll tack it on to the front of this one: in normal US parlance, a vessel that conducts dredging operations is called a "dredge." Every comment referring to a "dredger" is like fingernails on a chalkboard inside my brain. (Not you, Randomized, Controlled.)
Answering your question: Efficient shipping requires a deeper channel than normally exists naturally, and a dredge is used to create a channel of the desired dimensions. And, of course, since it's not a natural river bed nature keeps trying to make it on...
The Port of Long Beach, whose problems instigated the post the OP is responding to, publishes container movements here:
They're up ~150,000 TEUs since early last year (just plotting the "total" column and eyeballing it). IIRC, most containers are 40', so that's somewhere around 75,000 more containers per month. Note that this it both inbound and outbound, full and empty.
There's enough uncertainty in both of those figures I wouldn't take it to the bank, but it shows that shipping volumes have increased at the port.
"According to Wikipedia": the three most trusted words in information!
In all seriousness, according to the talk page it's a UK/US language difference: https://en.wikipedia.org/wiki/Talk:Dredging#dredge_versus_dredger I didn't actually know that before, and I'm glad I actually added the "US parlance" qualification to my last comment, out of the vague wonder that there might be somewhere it turned out to be used.
I will say that this is a pretty firm usage if you're in the US, though. I've only tangentially worked with dredges (I've been on board ... (read more)