Denver’s project to ensure at least some water for fish in a 40-mile urban stretch of the South Platte River — even during the winter low-flow months when people practically drain it — is gaining momentum.

A fundraising goal has been met to buy space in Chatfield Reservoir, southwest of Denver, to store an “environmental pool” of water – about 500 acre-feet (163 million gallons), Denver Water officials confirmed last week.

Starting next year, state aquatic biologists plan to release that water strategically, concentrating on 65 or so low-flow days each year. The South Platte still will be one of the world’s most tightly controlled rivers, unable to be a natural river that meanders through a flood plain moving sediment. But biologists say a dedicated flow for ecological purposes will allow some bending within the engineered channel through metro Denver.

Mayor Michael Hancock has declared that all great cities, including Denver, are defined by major waterways. “The South Platte River is a central feature of our landscape, and we are committed to keeping it flowing through the year for the benefit and enjoyment of all of our residents,” Hancock said recently in a prepared statement.

Water releases will begin “after the completion of the Chatfield Reallocation Project,” Denver Water officials said, with the water moving from Chatfield through a Colorado Parks and Wildlife fish hatchery. Fish grown there, including rainbow trout, may be used to stock river pools where fish currently struggle to reproduce on their own.

Storing water at Chatfield, built for flood control but now in the process of “reallocation” for water supply, costs $7,500 per acre-foot (325,851 gallons). Denver Water officials agreed to spend $1.8 million and match 19 contributions made by metro county and municipal governments, the Greenway Foundation and the Walton Family Foundation. “The pledge drive was successful and complete,” Denver Water spokeswoman Stacy Chesney said.

Metro Denver grew up around the South Platte floodplain, with industrial plants and discharge pipes spewing contaminants into the river. But today, kayakers, cyclists, anglers and others flock to the river. Elected officials around an increasingly dense-packed metropolis face rising demands for more water, cleaner water and green space for people and wildlife.

For more than 20 years, conservationists have worked at restoring the South Platte so it could sustain aquatic bug life and fish – even trout. This requires curves, pools – and healthy flows of clean water.

Denver Trout Unlimited president Scott Schreiber said purchase of the 500 acre-feet “benefits our ecosystem, especially the aquatic life that will now flourish as a result of these additional flows.”

The Colorado Water Conservation Board will serve as the owner of the water held in Chatfield for environmental purposes. Water rights owned by the agricultural Central Colorado Water Conservancy District are being used to create that pool.

Aquatic biologists say that, by putting more water into the river, river managers can mimic natural flows, lost after the channelization of the Platte following a ruinous 1965 flood that destroyed structures built in the floodplain.

More water likely would help fish.

Anglers along the South Platte through Denver have focused on carp. They’ve also found smallmouth and largemouth bass, bluegill, suckers, catfish and walleye. There’s growing enthusiasm for trout, with fishermen reporting recently that mayfly-imitation nymphs will entice them.

Beyond Denver, farmers use much of the treated wastewater that cities inject back into the river north of Denver. In northeastern Colorado, the South Platte periodically runs dry, though some surplus water still reaches Nebraska.