Rinse the rhubarb and slice into 1-inch pieces, discarding any leaves. Place the sliced rhubarb in a large pot with the water. Bring to a boil over medium-high heat, then reduce to a simmer and cook until the rhubarb is very soft and falling apart, about 10 minutes.

Line a colander with a double layer of dampened cheesecloth, or use a jelly bag. Pour the cooked rhubarb into the lined colander set over a large bowl. Let the juice drip through for 1 to 2 hours. Don't squeeze the bag if you want a clear syrup. You should get approximately 4 cups of juice.

Pour the strained rhubarb juice into a clean saucepan and add the sugar. Heat gently over medium heat, stirring until the sugar dissolves completely. Bring to a gentle simmer (around 180 to 190°F) but do not boil. Remove from heat and skim off any foam.

Ladle the hot syrup into prepared jars, leaving 1/4 inch headspace. Wipe the rims clean with a damp cloth, apply lids and bands, and tighten to fingertip tight.

Lower the filled jars into a prepared water bath canner, making sure they're covered by at least 1 inch of water. Process for 10 minutes for pints or half pints, adjusting for altitude.

Turn off the heat and let the jars stand in the canner for 5 minutes before removing them. Place on a towel-lined counter and let cool undisturbed for 12 to 24 hours. Check seals before storing. Refrigerate any jars that didn't seal for immediate use.