Sorry to be late to the party: been busy with wine!
First, are you sure it's Riparia? That's a pretty narrow category of grapevines--I'm more curious than anything. In any case my recipe is cribbed from my friend Jack Keller, who knows more about native grapes than anyone I know.
For five gallons
- 50 lbs Grapes
- 7-10 lbs sugar
- 3-1/2 tsp pectic enzyme
- 1/4 tsp potassium metabisulfite
- Acid blend (see below)
- 5 tsp yeast nutrient
Also about 2 to 2-1/2 gallons of topping water and yeast of your choice (can never go wrong with EC1118)
Crush and destem the grapes, and pull off enough juice to get an sg reading. Add sugar to the must (use dextrose because it dissolves easily) to bring the sg to 1.090--one pound of sugar per gallon will add 1.042 sg. Add enzymes and sulphite, stir hard and pitch yeast.
Punch down cap twice a day for 7 days, or until sg is below 1.010, press off grapes and top up pail to 5.5 gallons (losses will happen). This bit is important: for every gallon of water you add, add 2 lb of sugar--measure carefully, because you need this for alcohol content.
Measure the TA: if you picked after frost, you may need to add a bit due to watering back. Get in a range of 5.5-6.5 g/l. Use tartaric, because you can cold-stabilise later and drop the pH a bit more if it's an issue.
Let sit for three more weeks to finish fermenting and drop gross lees. Rack into a 5-gallon carboy, add 1/4 teaspoon of sulfite and top up if necessary. In three months rack again, add fining agents, adjust sulfite to a pH-appropriate level, and in a month or so when it's clear, go to the bottle.
Riparia isn't to my taste, but I will say that well-aged examples (3-4 years) are a lot more palatable than anything younger. However, if you're going to back-sweeten, you can drink it as young as you like.
Hope that helps out.