Figured it was about time I finished off the watercourses on my terrain tiles. If you've seen these in a couple of recent games, you'll have noticed the river beds are painted but not watered - they look ok from a distance but not so great close up.
For those wondering, the river tiles are built on two 25mm tiles, with the water courses cut out of the top one. Their cemtrelines are 150mm in from the corner of the 600mm tiles, and at the edges they're exactly perpendicular for the first few mm. The cuts are 50mm wide at the bottom and 100mm wide at the top, so the sides should in theory be exact 45º angles and everything should match up. The river bed is them textured with a roughly 5mm thick layer of very lightweight ready-mixed Polyfilla (aka Woodland Scenics Foam Filler only much cheaper).
What I've then done is sealed the ends of the cuts off with good quality gaffer tape (not the £2 cheap rubbish with no stiction), and marked a faint line on the inside 17mm below the top, as a rough water level. The water is, (as for the marsh I made), Javis Scenic Water, which comes in a nice applicator bottle which is actually useless when you're making this amount of water, so I unscrewed the lid and poured, carefully, having ensured it was level with my trusty Tiltmeter iPhone app. (You'll notice that as a precaution against seepage there's newspaper under both ends!).
Now, we just shut the cats and family out of the room for 24-36 hours and leave it to dry.