Colorado's complex system for appropriating water - which was shaped in part by disputes over Poudre River water - will come into play if Glade Reservoir is built.

State water law is based on the concept "first in time, first in right," also known as the doctrine of prior appropriation.

Whoever has the longest-standing claim on a specific amount of water and can put it to beneficial use - as in supplying it for domestic, agricultural or industrial purposes - has the first right to use it.

Water rights retain their seniority when they are sold or traded. Poudre water that is claimed but cannot be used or stored flows out of state.

One of the rights that would be used to divert water into Glade is considered a "junior" right. It could only be taken during times of high flows on the Poudre, typically during the height of spring runoff also known as the "June rise."

The reservoir also would take in water owned by two irrigation ditch companies that already draw from the river.

Glade's water right dates to the 1980s and a proposal to build a dam within Poudre Canyon. Senior rights on the river date back to the 1870s. The city of Thornton has a conditional claim to 36,000 acre feet of Poudre water that is junior to the Glade right.

If the reservoir is not built, Thornton could at some point exercise its right to the river's water, said Bill Brown, a Fort Collins water attorney and longtime member of the Northern Colorado Water Conservancy District board of directors.