A section of Tung Chung Stream damaged by illegal excavation work has had its natural charm returned after two months of intense restoration.

The quiet stream on Lantau Island was thrust into the headlines late last year when unauthorised works left part of it damaged and scarred. Some boulders and rocks were removed from the stream.

The Government set up a taskforce to return the stream to its natural state, chaired by the Environment, Transport & Works Bureau. It worked with a specialist group, including representatives from the Advisory Council on the Environment and major green and interest groups, to determine the best way to approach the job. Work began in February and finished in April.

Back on stream: The damaged section of Tung Chung Stream has been returned to its natural state, and many free-flying insects, like damselflies, can be seen again.

Boulders strategically placed

Agriculture, Fisheries & Conservation Department Nature Conservation Officer Ho Ching-yee said the works aimed to restore the stream's physical settings.

"The works involved removing an unauthorised access road to restore the depth and width of the stream and stabilising the stream banks with large boulders and biodegradable geotextile. Other boulders were then placed along the stream to restore the streambed," the officer said.

During the laying of boulders, pools and riffles, which are the shallower, faster moving sections of a stream, were created to provide various habitat types for different stream fauna.

HK University Ecology & Biodiversity Department Assistant Professor Dr Billy Hau, a specialist group member, said creating these habitats was not easy as all streams become very dynamic in summer due to heavy rains.

"Therefore, we suggested the workers strategically place some big boulders in the stream. Those boulders will stay there even during flood, stopping the water from washing smaller living things away. Meanwhile, smaller stones will move with the water to form riffles and pools."

Dr Hau said the specialist group is satisfied with the work.

"So far we feel the result is rather satisfactory and encouraging. All the designed big boulders are there, pools are created and riffles are found. I think the works are quite successful."

Although there were some reshuffling of pools and riffles in the stream because of a torrential rainstorm in early May, the resulting profile appears even more natural.

Ecological recovery happening

So far, more than 20 different kinds of fresh water fish and macro-invertebrates have been seen in the repaired stream section. They include the rare Beijiang thick-lipped barb, predaceous barb, Chinese barb, mosquito fish, swordtail, mayflies, dragonflies, caddishflies and damselflies.

"The vertebrates in the streambed are recovering more slowly than things like fish or free-flying insects. The macro-invertebrates in the streambed are still rather small and I suspect they were reproduced in the upper stream and have flowed with the water and established down here," Dr Hau said.

"For fish, the recovery seems to be faster. A few species have already re-colonised in pools and riffles here. Four to five species of dragonfly can also be found. I think the recovery is happening and the result is quite good."

Dr Hau said it is difficult to predict how long it will take for the stream to fully recover.

"Judging from the progress right now, I believe the recovery would be as much as half of its original [state] in the next summer. Whether it can be fully recovered, it is difficult to say. I would think in three to four years, the eco-system will more or less resemble the original one. There will be species changes, I believe, because the impact is quite drastic but the system will recover in about three years."

Stream supports indigenous freshwater fish

Tung Chung Stream is an ecologically sensitive habitat of conservation value. It is about three kilometres long and supports over 20 species of indigenous freshwater fish.

Late last year, a stream section of about 330 metres was found damaged by unauthorised works. An access road of about five to 15 metres in width was built alongside the damaged stream section, and some boulders and pebbles on the streambed were removed.

The Government then demanded Tung Chung Rural Committee which was in relation with the case to undertake restoration works to return the damaged section to its original state.

The department will monitor the repaired stream quarterly for at least two years to examine the progress of its ecological restoration.

For photos of the repaired stream click here.