Here's the breakdown of how I draw lineart for swamps.
1. The rivers
Swamps are often around a river - if this is the case, then begin with the river at the heart of the swamp. Unlike most rivers which usually run for miles without branches, in a swamp I add lots and lots of tributaries feeding into the main river. This indicates the water draining in from the wider swamp and helps to define the borders of the swamp. Because I'm drawing a 3/4 style map here, I emphasise the horizontal spread of the rivers over the vertical spreads.
If your swamp isn't connected to a river, then ignore this step.
2. Tufts of swamp grass
Add in tufts of grass throughout the swamp. 2-3 lines spiking up from the ground should do the trick.
3. Horizontal lines
Here we really specify the area of the swamps. I add horizontal lines and ripples to imply the surface water in the bog. The lines don't meet up with the tufted grass - that separation helps keep the texture clean rather than messy. Finally I add a rippled line around the edge to define the limits of the swamp.
I hope that helps! I'll put together a parallel tutorial with colour hopefully later this week.
For more tutorials check out the full list on the blog: [link]