This Japanese drawing marks point A and B as «Three hundred hectare territory of dispute». This seems to be the origin of the Sino-Russian conflict at this area. 
The lake is Lake Khasan. The town at the bridge is Khasan. China is west of the lake, and Russia is east of the lake.

According to the 1991 Sino-Soviet Border Agreement, 3 km² (1 mi²) of territory would be transferred to China, and Chinese ships would gain the right to navigate the Tumen river. The territory transfer would connect a previously enclaved piece of Chinese territory to the rest of China. This portion of the agreement stirred up some controversy among some Russian officials from Primorsky Krai, as they felt that direct Chinese access to the Sea of Japan (through the Tumen River) would decrease the economic importance of Vladivostok and Nakhodka. Other arguments protested at potential pollution from Chinese economic development, the possibility of Chinese military vessels navigating the river, and the presence of a Russian cemetery commemorating the Lake Khasan Incident in the area. Finally, in June 1997, the Russian side proposed a resolution which would divide the disputed territory in half. This was accepted by China in September of the same year, and in November, when the border demarcation work was declared finished, 1.6 km² (0.6 mi²) were transferred to China, and 1.4 km² (0.5 mi²) were retained by Russia. The Lake Khasan cemetery remained on the Russian side, and the Chinese officials underwent informal agreements to not build a port along the Tumen River. ¹⁾

Serious border demarcation negotiations did not occur until shortly before the end of the Soviet Union in 1991. In particular, both sides agreed that Damansky/Zhenbao Island belonged to China. (Both sides claimed the island was under their control at the time of the agreement.) On October 17, 1995, an agreement over the last 54 kilometres (34 mi) stretch of the border was reached, but the question of control over three islands in the Amur and Argun rivers was left to be settled later.

In a border agreement between Russia and China signed on October 14, 2004, that dispute was finally resolved. China was granted control over Tarabarov Island (Yinlong Island), Zhenbao Island, and approximately 50% of Bolshoy Ussuriysky Island (Heixiazi Island), near Khabarovsk. China's Standing Committee of the National People's Congress ratified this agreement on April 27, 2005, with the Russian Duma following suit on May 20. On June 2, Chinese Foreign Minister Li Zhaoxing and Russian Minister of Foreign Affairs Sergei Lavrov exchanged the ratification documents from their respective governments.

On July 21, 2008, Chinese Foreign Minister Yang Jiechi and his Russian counterpart, Sergei Lavrov, signed an additional Sino-Russian Border Line Agreement marking the acceptance of the demarcation the eastern portion of the Chinese-Russian border in Beijing, China. An additional protocol with a map affiliated on the eastern part of the borders both countries share was signed. The agreement also includes the return of Yinlong/Tarabarov Island and half of Heixiazi/Bolshoi Ussuriysky Island. ²⁾

According to a Chinese blogger in September 1997, China and Russia in the case of mutual concessions, were in the final completion of demarcation negotiations. The largest discussion theme between the two sides was regarding the mouth of the Tumen River area at Khassan. Russia had promissed China access to the Sea of Japan at the lake southwest of the Russian town of Kraskino, but in the middle of the 1990s the Primorsky Duma denied any border change because "it was not in the interest of the Russian Federation". The Chinese side from the overall situation of Sino-Russian relations, agreed to make some adjustments – Russia should handed back Sakhalinsky Island to China as compensation. In early November 1997, the third time in President Boris Yeltsin during his visit, China and Russia issued a joint communique that the leaders of demarcation work completed on schedule. Sakhalinsky Island had become a part of China. ³⁾

Due to changes of the river path since the 1860 Peking Treaty the land between Hunchun and Fangchuancun (near the tripoint area) was washed out in the river and the Fangchuancun became fragmented from dry-land connection with the rest of China. ⁴⁾

4. Part of Soviet Topographic Map K-52-058 (from about 1980-84).