James: They are used as neighborhood walking trails and even horsetrails as well as by the landowners and RG Conservancy District employees; most of the ditches back up to residential areas surrounding the little remaining farmlets and ranchlets. The trails vary in width: some are doubletrack, some so narrow that, nervous Nellie as I am, I get off and walk.
The water is allocated according to an ancient and as I understand it unique code of law dating at least to late 16th century; perhaps pre-Conquest since the Pueblo people started the system that is neither US common or statute law nor Euro-Napoleonic law; I read that it originated in Arab countries (with the word "acequia"). You'll see a ditch full for a week or so, then bone dry the next. The ditches are interspersed with hand-operated sluice gates, some modern and steel, some old, decrepit, and wood.
As I understand it, the paths are by law open for public use even though some landowners block them up; my brother was telling me yesterday of a new fence just erected to keep out homeless campers who pose a serious fire danger.
And yes, at least 30 miles within a 5-mile radius of my house, all to the east (I'm on the westside of the River) without backtracking, and starting almost literally out my front door; tho' to use them all in a single ride you'd have to backtrack in many places, and add pavement to access them all. I did a mental inventory last night and yep, 30 is the minimum. Yesterday's ride was 15 miles including backtracking and pavement gaps but mostly ditchbank.
This photo is not mine but I could supply any number myself.