Here's a starter on the subject of suitable flies for the Alness, taken from the Virtual Salmon Guide section of the Novar Fishings website (where you will also find comprehensive information on water levels):
Low Water 40-55 cm on the SEPA gauge (0 to +6in above summer low)
0.5" brass/copper bottle tubes, size 12-14 doubles
Low water generally means small flies, and shrimp patterns should not be too heavily dressed, but don’t rule out tubes; small brass/copper bottle tubes in particular. These can be very effective is deep, fast runs at the heads of some pools. And you can also try flashy, heavy flies, cast into the depths of the deepest pools, and fished with a ‘sink and draw’ retrieve.
Mid Water 55-70 cm (+6in to +1ft)
0.5" to 1" aluminium/brass/copper tubes, size 10-12 doubles
Anything goes at these levels; doubles and tubes are both effective on floating lines, indeed you may want to change fly type from pool to pool.
High Water 70 -100 cm (+1ft to +2ft)
1" to 1.5" brass/copper tubes, size 6-8 doubles
At the lower end of this range, a 1 inch tube on a floating line or a size 8 double on a fast sink polyleader should be effective, but at higher levels, you may need a polyleader in combination with a brass/copper tube. Even in high water, there is no need to get down to any great depth, just a few inches down can make the difference, and with this set-up, your fly will likely fish progressively deeper as it swings around to your bank. These are also good water levels to try stripping a Collie Dog or Monkey across the pools, the tails in particular if the spate is receding. If the water is rising and starting to colour, try a bright silver or gold-bodied fly.
Very High Water >100 cm (> +2ft)
1.5" to 2.5" brass/copper tubes
At levels just above + 2ft, you can still fish a conventional double on a fast polyleader or sink tip, as long as you have a short leader, but you will do better with a big tube; 1.5 inches is normally sufficient, though you can go bigger especially in late season. If you are struggling to cast a big tube on a sunk polyleader/tip, then try a 1 inch shrimp pattern such as an Ally’s Shrimp or Cascade, which with its long tail has more presence.
Below is our virtual fly box, showing the types of flies and patterns effective on the River Alness.