China has built a new dam on Fuyuan waterway, a small pathway south of Heixiazi Island and a by-pass before Heilongjiang (Amur) and Ussuri River 'formally' meet.
As illustrated in the map below. The yellow stroke is where the dam is located. Heilongjiang is polluted when the tributary Songhua River merges upstream (from west), But Ussuri River is clean (flows from SW). After the dam is built, Heilongjiang water would not meet Ussuri water until point B, the water between points A and B will remain clean, which is where Russians water supply source is.
The building of the dam is suggested by the Russians, according to some report. Fuyan water is very shallow (0.9m at tis deepest, 0.3-0.4m on average).
Meanwhile, there is also rumour that the Russians worry about the dam building will dry up the Fuyuan waterway and hence connecting Heixiazi will Chinese side of the bank. The rumour is unfounded, as we can see form the map, the western portion of Heixiazi belongs to China upon recent border negotiation and both sides of the Fuyuan waterway will be inside China border. (The actual hand-over will happen in 1-2 years)
Some history of the border dispute: in 1929 USSR took Heixiazi and Yinlong Islands, plus a number of other border territories from China. In 1964, both sides agreed on principle that demarcation would be based on shipping route, which means China would get back both islands. However, no formal treaty has been signed and the relationship between the two countries deterirated. Negotiation in recent years results in current compromise, where China would get about 174 sq km (about 50%) out of a total of 350 sq km. In addition, China will have the nagivation right to go through the other sides of Heixiazi to connect sail from Heilong Jiang to Ussuri Jiang.
Related note: I expect the Kurils dispute between Russia and Japan might end up with a similar deal.
The map above is at the NE tip of China, see the insert (lower right corner, and a small rectangle in the larger map above) of the map below for its location.