Half of the area below the dam is closed this week, while work crews remove seven ailing Lombardy poplars. Seventeen were taken out last year, and 54 London plane trees, lipstick maples (which turn a brilliant red in autumn) and Canadian pin oaks were planted.

Senior ranger, Murrumbidgee corridor, Darren Roso said more trees would be planted, including more London planes and Fastigiate English oaks.

"These species are similar in appearance, but are more drought-tolerant, longer living and easier to maintain. Oaks, in particular, put on a spectacular autumn leaf display, much like the poplars," Mr Roso said.

"We try and diversify the species we have in our reserves, because every once in a while you get a disease that comes through and affects one species, but not all the species," he said.

A quote from English foresters he came across during his studies has stuck with Mr Roso: "An oak has 300 years to grow, 300 years to live and 300 years to die. That is the kind of longevity and forward thinking we are looking at. Everyone must have foresight, otherwise we will be taking out trees every 80 years."

In years to come the trees will shade thousands more Canberrans. When he began counting arrivals at Cotter Avenue in late summer 1991, Mr Roso estimated 46,000 people had visited, assuming two people in each car.

"We get about 200,000 to 300,000 people a year now. The record, 264,000 people, was in 2002. Then the [Canberra firestorm] happened. it was closed for a year, then the Cotter Dam construction closure happened."

Mr Roso said most Canberrans had happy memories of visiting Cotter Avenue, and its inviting, cool grass and lines of trees.

"All the lawns have been cored. As soon as this tree work is finished we will top dress the lawn and fertilise it and it will be ready for summer," he said.

The poplars are about 80 years old and in recent years have been in poor health due to their age.

A landscape plan is being produced for the Cotter Avenue that will recommend replacement species and layout. The plan is expected to be completed next month.

Mr Roso said tree planting was happening across the broader Murrumbidgee River corridor area.

"In total we expect that approximately 20,000 native and exotic trees will be planted in the Murrumbidgee River corridor this year. Many will be replacing older or unsafe trees. This sort of tree replacement work is vital in preserving our parks and reserves for future generations," he said.