Aerial photo taken on Sept. 7, 2018 shows the scenery of Heihe River wetland in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Geese swim at Heihe River wetland in Gaotai County of Zhangye City, northwest China's Gansu Province, Sept. 7, 2018. (Xinhua/Tao Ming)

Aerial photo taken on Sept. 7, 2018 shows the scenery of Heihe River wetland in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Aerial photo taken on Sept. 7, 2018 shows the group wedding ceremony at a wetland park along the Heihe River in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Photo taken on Sept. 7, 2018 shows the group wedding ceremony at a wetland park along the Heihe River in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Aerial photo taken on Sept. 7, 2018 shows the scenery of a wetland park along the Heihe River in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Aerial photo taken on Sept. 7, 2018 shows the scenery of Heihe River wetland in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Aerial photo taken on Sept. 7, 2018 shows the scenery of Heihe River wetland in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

Aerial photo taken on Sept. 7, 2018 shows the scenery of a wetland park along the Heihe River in Gaotai County of Zhangye City, northwest China's Gansu Province. (Xinhua/Tao Ming)

People visit the Heihe River wetland in Gaotai County of Zhangye City, northwest China's Gansu Province, Sept. 7, 2018. (Xinhua/Tao Ming)