If you are using the same filter that Aggro mentioned years ago, and that I am using, it is the teal coloured woody-stringed matt or thatch type. It should be cut into small disks, and then teased apart to make think wafers of the stuff. All of it can be used, except for the corner stuff you cut off to make the disks, although some of those can serve as toppers to the armatures.
I sharpen the armatures to points, rough up their sides so the trunk looks like bark, and then I spray or hand-paint the whole dowel. I then apply the wafers, largest towards the bottom, etc. and spaced out appropriately. Then I use quick bursts from a flat brown spray can of paint to lightly colour the wafers, top and bottom. It is only than that I spray the wafers carefully and sprinkle on the ground foam.
Last thing is to wash the lower trunk so that the bark creases get highlighted. I also stain to adjust the tone of the bark from too brown to more greyish....whatever seems necessary.
For the water 'bed, I would resort to a putty. You can mix a gooey putty and slather it into place. Later, sand it to smooth it a bit. Then seal it with your acrylic paints to generate the depths look you want. Then add water. Make sure you use a minimum of two thin pours of your epoxy material...your envirotex, and allow 24 hours between pours. Cover each pour with stiff cardboard to keep dust from settling on it until it is fully cured. If you don't want the glassy surface, get some gel "gloss medium" act craft/arts stores and use a painter's brush on its side to stipple a thin layer of it across the surface of your envirotex.
One other thing...you should consider staining your envirotex. A pour comprising near 1/2 cup of the stuff would take about 1/2 drop of a yellowy-green acrylic paint. If you want it to be somewhat opaque, as river water often is, then consider adding about 1/2 tsp of plaster of paris as well. Don't be alarmed at any foaming or whatever...go ahead and layer it and spread it with a piece of sprue or scrap stripwood. Let it cure. It will spread and the foaming should go away. It did for me.
Also, to build confidence and skill, do a practice run of the entire series of steps on a piece of scrap plywood. When you see that it can be done, with some obvious adjustments for your tastes, you can go to the layout.
Here is how it turned out for me. I can't begin to tell you how pleased I am with the results. It looks so much like the water I had in mind that I danced a jig the first time I imaged it. Note, though, that it is very green standing over it...you see reflected light, something I had not considered. I was lucky. So, instead of my full drop in something less than half a cup of envirotex, try the half-drop first.
Here is what I had in mind: the South Thompson River in southern BC.