Here's a Shot taken with a B&W 10 Stop filter, What do you think?
1D Mark 4
17-40 @ 33mm
you asked, so...I do a lot of LE...and one thing I always have at the front of my mind when considering a scene for an LE,...what will the image gain in dynamics from shooting a 1 minute exposure or in many of my cases longer than that?
In this scene, the only thing you stand to gain as I see it, is movement in the trees, and an even harder time balancing your exposure from the lights to darks. Those water cascades are going to look almost the same with a 1-3 second exposure as they do now, not a lot of white water after them, no swirls or eddies, so for me, the little bit of details lost in the trees from the long exposure hurt more than you gained by going out to 1 minute. And with the 17-40L, a good as it is, the edges vignette, and this is exaggerated by using a Big Stopper so you end up with dark corners.
I might have also tried to make the cascades the main element, get closer and fill the frame more with them, as they are now, they are small in the frame...looking at your Flickr page, you did that...and I like that shot much more, it eliminated the dark areas, made the cascades a bigger part of the frame...it still has the movement in the trees from the LE...its more evident in that one.