Help needed to keep away weeds coming from a creek

Question

The Problem:

There is a small creek in my backyard and it's a nightmare to maintain. A lot of weeds grow there and spill over to my land, some of them growing up to three or four feet tall and are very dense. The portion of my land that is adjacent to such creek is 265 feet long. This is a warm climate (Puerto Rico to be exact), so they will always grow there. My goal is to keep them at bay, and not entirely eradicate them (I know that is impossible) Here are some photos so you can have an idea:

(Some description: the metal fence you see there belongs to a neighbor who is beyond the creek. There is no fence separating my land and the creek. And then you can see the weeds, some of which include Castor oil plants; they grow a lot here: http://en.wikipedia.org/wiki/Castor_oil_plant)

(Description: a better view of the creek with some pooled water from the last time it rained. The water doesnt get high and there is a slope in which I can plant more stuff.)

(Description: this is the part where it is the worst. Here, the weeds have grown VERY dense.)

What I Have Tried:

I have been hiring people to chop it down, but I dont want to spend money on that every single month. I have tried to lay down one of those landscape fabrics, but that was entirely futile. And I dont want to spray herbicide... it's a creek, and I dont want to pollute. Any research that I do online is for smaller problems, like pulling them out (already tried that, believe it or not), or spreading bark mulch (its too much, 265 feet, and the water of the creek will eventually swipe it), etc.

What I Plan:

My next idea will be to try and plant some tenacious shrubs that can compete against those weeds and keep them at bay. But which ones can handle this (almost everything grows here in this climate)? I had in mind Bougainvillea (http://en.wikipedia.org/wiki/Bougainvillea) since I have heard they are very hardy.

Any other idea is really welcomed.

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Additional Comments:

These comments originated from the responses below.

1. My main reason for battling the weeds is that they look ugly and propagate too FAST into my land, in which I'm starting a vegetable garden. Sure, not a life-threatening situation, so I will reconsider my stance a bit. Note that I want to keep them at bay, and not entirely eradicate them (I know that is impossible)
2. The water doesn't get too high, even in torrential rains, which happens once every one or two months. And yes, there is a gradual slope in which I can put plants. I have included two additional photos.
3. Yes, the soil inside my land is not the best. Some time ago, an excavator went through it to clear the ditch, and it compacted the land and also toiled the soil in the creek a lot (since I have started a vegetable garden, I won't allow any more excavators or heavy machinery). But the actual soil at the edge of the ditch is somewhat loose and easy to toil and with good drainage.

Answer 1

Reading your question, I'm moved to ask, "Why cut the weeds in the channel at all?" You mow to the edge, and could much more easily manage weeds on the level ground. The weeds, though tall, do not seem to block any kind of view. Finally, the dense vegetation in the ditch is an excellent buffer for water entering from your property and will slow the movement of water through the channel. Slowing movement of water is generally desirable in such channels, though you don't want to slow it down so much that it ponds up over the banks.

But assuming you still want to deal with the weeds:

Your plan is really the only good way to go about it. You've got to get something in there that will out-compete the weeds.

One issue you are sure to have to deal with is the existing weed seed bank. I'm glad you don't want to use herbicide, but that limits your ability to deal with the weeds. You also won't want to till along those banks. You could try prepping the site by solarizing the soil after cutting the weeds to the ground, but this will have limited use as it will not be very effective against perennial weeds.

Ultimately success will depend on what you plant there. Bougainvillea sounds like it would be a decent choice - just make sure you get a shrubby and sprawling variety that doesn't want to climb. Depending on how how high the water gets and how often it gets there, you might be limited in how far down the slope it will survive. From what I read, this plant needs good drainage also. Judging from the foreground of your photo, I'd guess the soil is somewhat compacted and not well draining. If you do go this route, you'll still want to manage weeds until the Bougainvillea are established. Depending on how densely you can plant that could take several years.

Answer 2

I like That Idiot's answer. I'm simply going to state that if you have a weedwacker, bimonthly weed-whacking is effective in keeping existing vegetation under control. I don't see why you have to grow anything different. It should take about an afternoon maybe 5-6 times/year, and also a couple gallons of gas.