Fed, brushed and saddled, Paint, Princess and Pete are ridden to a vantage point overlooking a winding trail of fog that hides the Murrumbidgee River's timeless journey below.

Their riders Jo Sharp, Tamara Abed and Tash Page-Murray can barely be heard above the peewees, willy wagtails and carolling magpies.

Steam rises from fence tops, the backs of horses and feed bowls where big heads disappear to grind up a breakfast of mixed grains.

Off the Cotter Road, Riverview's 22 hectares allows agisted horses to run in herds, power up the hills and trot through a picturesque forest.

Owner Nikki Main says open spaces in the ACT for horses are limited, because many people want individual private paddocks. But on Riverview 60 horses are free to roam, safe in numbers to graze, and lie down.

ACT Endurance Riders Association president Simon Bain, who lives across the border in Yass, says he can ride for 50 kilometres in the ACT untroubled by cars or restrictions.

''The ACT has the reputation of being one of the best riding places in the world for the availability of trail rides.''

That freedom is becoming restricted, as the ACT government reins in horses in the mountains and Lower Cotter Catchment, a policy questioned by horse owners.

The ACT Equestrian Association has lobbied not to have horses excluded so broadly, without any discussion of why horses contribute more to dirty water in the catchment than cars, motorbikes or cyclists.

Riders in the Brookvale Ride, a national endurance event, say restrictions last year became so onerous they considered moving to NSW.

Organiser Gertraud Norton said since 2009 they had to do poo patrols to clear the course on the day or day after the ride to avoid manure washing into creeks.

Run over the October long weekend, the Snowy region is joining this year's ride, so organisers had to find a 120-kilometre course.

To avoid steep hills would have required going into the Cotter catchment. ''I contacted James from the Cotter catchment authority and he said, 'No. Don't think about it.'

''So we had to stick with the very challenging course.''

According to Territory and Municipal Services, until 2004 the Cotter Dam had not been used for domestic water supplies for more than 30 years.

Before work began on the enlarged Cotter Dam, the government restricted horse riding in the Lower Cotter Catchment to roads and purpose-built trails. The route connects Uriarra settlement with the Sherwood pine plantation.

Drawing water from the Lower Cotter Catchment also led to restricting off-road vehicle driving and fishing upstream to protect Canberra's water supply.

Horse riding is no longer allowed in the Brindabella Mountains. Within Namadgi National Park, horse riding is permitted on the National Bicentennial Trail, roads east of Old Boboyan Road and public roads in the park and the park's northern section.

ACTEW says the exclusion policies reduce the risk of animal-borne pathogens from entering reservoirs and drinking water catchments.