ACTEW Water chief engineer John Dymke will guide the public for the first time into the wall's gallery, under 900,000 tonnes of concrete.

Water monitoring instruments, outlet valves, drainage pipes and two sets of stairs leading to the top can be accessed from the gallery, which is generally closed to the public.

The tour is part of Cotterfest, which will celebrate the launch of a 2.5 kilometre walking trail from the valley floor in Cotter Avenue to the top of the new dam wall.

Journalist and author Richard Begbie will launch the trail, and tell the story of convict Garrett Cotter, after whom the river, dam and valley are named, and his friendship with Aboriginal leader Honyong, who led the young pioneer in 1828 to where Pierce's Creek meets the Cotter River, and grazing lands rich in native pastures. Friends of the Cotter convinced ACTEW to name the trail after Honyong. It will open with an indigenous smoking ceremony.

In the 1970s Begbie was a neighbour in the Naas Valley of Cotter's great grandson Bill Cotter, who kindled his interest in the friendship between Garrett, a young, slightly built but fiery Irishman and the big Aboriginal leader.

Begbie is researching Cotter's story for a novel and has spoken extensively to the Cotter family and local Aborigines, including elder Matilda House, a direct descendant of the chief's, to learn more of the story of Honyong, or Onyong, as he is sometimes called.

Honyong died in the 1850s and is buried on a hill above the Tharwa bridge. Begbie said he may have come from the eastern side of Lake George, and spent his adulthood in the mountains.

To his knowledge, there is little today to commemorate Honyong, aside from a plaque on the site of Garrett Cotter's original hut. (Begbie has uncovered different aspects of Garrett Cotter's origins, which will appear in The Canberra Times Forum section on Saturday).

Honyong's trail was originally graded for haulage trucks, taking rocks blasted from the edges of the valley for the dam's construction.

While the trail will remain open to the public, the gallery is unlikely to be opened again for some time.

It stands above 30 metres of concrete sunk under the base of the dam and enables maintenance crews to monitor the wall and undertake remedial grouting if ever needed.

Mr Dymke said concrete heated up substantially in construction. The dam's concrete was still 30 to 40 degrees and would take another 20 years to cool to the temperature of its natural surrounding atmosphere.

Inside the gallery people will see cracks engineered to manage leakages from the dam wall, and drains carrying off the water. Outside they will see three outlet valves of 200, 400 and 900 millimetres in diameter, for environmental flows of five to 40 megalitres a day.

The 900-millimetre outlet will be used about once a year to release 1200 megalitres.

''That simulates a flood. It's only for a day or two, it washes a lot of the algae out of the river, causes turbulence among the rocks and gets rid of a lot of the debris,'' Mr Dymke said.