Here is a real quick way to make nice, tapering rivers in Inkscape. I'm not sure what version I have (I always use the nightly windows unstable builds) but this require Live Path Extensions which are available in the last stable release.
Alternately, you could use the Path along path extension to get a similar effect, but not editable.
Start by drawing a long, horizontal taper. This will be your "pattern". The width a the right-hand size will be the width of the river mouth. My lines here aren't straight, they have a slight curve on them. select this and hit ctrl-c (or edit->copy):
Now draw the river you want. Start at the origin of the river and end at the mouth!. I use the pencil tool, then simplify the shape using ctrl-l (path->simplify). I then use the edit path by modes tool, select all the nodes (ctrl-a), and click the "make selected nodes smooth". here you can see the curce I drew with smooth node handles:
With the river curve selected, open the Path Effects dialog (path->path effects). From the "Apply new Effect" dropdown, pick "Pattern along Path" then click "Add". Click the clipboard icon to paste the pattern we had copied back in the first step on to the path. For rivers, select Single, stretched. You can click on the Node Edit tool in the path effect dialog, and you will be able to change the pattern dynamically, seeing how it looks on the path:
The nice thing about pattern along path is that the original path (the river) can still be edited, and the pattern will just follow.
Draw a tributary (again with the pencil tool, then simplify and smooth nodes), keeping the direction in mind again. Repeat the last step. Here I am editing the pattern nodes to make this segment more narrow:
To combine the rivers, select them all, and set their path and fill to "None", then group them together. Set the group fill to the colour you want (I picked cyan to demonstrate). Duplicate that group, and set the duplicate's stroke to a darker colour. I used blue, and 3 pixels. Now push the duplicate group below, and the whole river is nicely outlined (you could also blur the outline for a softer look):
It is just as easy to add lake shapes to the group before setting the group stroke and fill to make lakes and rivers with continuous shoreline strokes.