The groundwater system is a very efficient underground reservoir, since underground water does not evaporate. Seepage from ditches has built up the groundwater reservoir in the Platte basin over the last 150 years. Now, water from this groundwater reservoir flows back into the Platte further downstream and supplies ditches out east.
Back in 1859, the Platte River didn't have enough water in it to fill any ditches out east. As upstream irrigators near Boulder and Fort Collins saturated the aquifer, return flows to the Platte grew until there was enough water to supply ditches lower down on the Platte. This is why major canals east of Greeley generally have junior appropriations relative to most ditches closer to the Front Range.
Photo top courtesy of Boulder Open Space and Mountain Parks.
