Debbie and I visited yesterday because we were wondering about the same things. It appears to be a part of the Presidential Library grounds, where a small wetland area has been made accessible with a nice boardwalk and interpretive signs talking about the value of wetlands. There are also some educational signs specifically about how litter washes into our waterways. The trash trap is still there, and appears to be functioning as designed. Some amount of trash might bypass the trap during high flows, but I think the majority of trash seen in the wetland is a combination of what blows in and what comes in via other untrapped drains. Some of it might have been thrown from the boardwalk.
Also found this link:
http://www.littlerock.org/citymanager/billclark.aspx" onclick="window.open(this.href);return false;
Based on that, it appears to be under the ownership of the Little Rock City Parks. It does seem "odd" that they can mow and manicure the lawn, carefully prune and maintain lots of nice shrubbery, pick up litter on the vast majority of the grounds, and yet call on volunteers for this particular effort.
Two possible explanations come to mind:
1) This one could be really more of an educational outreach intended to increase public awareness of the area, and maybe educate those who didn't know where that stuff you throw out on the road or into the storm drain ends up. As far as the trash in the pond goes, its unsightly but the amount there is something 3 or 4 of us could knock out in about an hour, if that long. So my thoughts are surely, they must have something "bigger" in mind, which leads to 2)
2) The link says the wetland area is 13 acres; looking at it on Google Earth the little pond and boardwalk area is only about 1.5 acres. So if the effort is to take in some of the "wilder" areas near the Center including the big island just downstream of the old railroad bridge and try to clean up that, then I could see it going beyond the "normal grounds upkeep" and becoming something more akin to Keep Little Rock Beautiful efforts to hit larger areas like the Great Arkansas River Clean Up we recently participated in.
The pond area and the island can be accessed via direct connection with the Arkansas River, so two options present themselves to anyone wanting to participate by boat: a) Carry your boat down from the parking lot, which might present some issues in finding a good place to launch, or b) Launch in NLR at the large boat ramp near the boat house (almost under I-30) and paddle (or motor) 0.3 miles across the river to the Presidential library.
I'm going to be out of town and unable to participate on the day of the event, but may do something on another day.