Question: Battle of the Barrier Forts was fought along the river that has what total length?
Context: The Israeli West Bank barrier or wall (for further names see here) is a separation barrier in the West Bank or along the Green Line.  Israel considers it a security barrier against terrorism, while Palestinians call it a racial segregation or apartheid wall.  At a total length of 708 km upon completion, the border traced by the barrier is more than double the length of the Green Line, with 15% running along it or in Israel, while the remaining 85% cuts at times 18 km deep into the West Bank, isolating about 9.4% of it, leaving an estimated 25,000 Palestinians isolated from the bulk of that territory. The Moselle-Line was the designation of a line of French barrier forts ("forts d'arrêt") on the upper Moselle between the fortresses of Épinal and Belfort. The Battle of the Barrier Forts (also known as the Battle of the Pearl River Forts) was fought between American and Chinese forces in the Pearl River, Guangdong, China in November 1856 during the Second Opium War.  The United States Navy launched an amphibious assault against a series of four forts known as the Barrier Forts near the city of Canton (modern-day Guangzhou).  It was considered an important battle by the British whose interest lay in capturing Canton. The Treaty of Den Haag (also known as the Treaty of The Hague and seldom known as the Hedges Treaty) was signed on May 16, 1795 between representatives of the French Republic and the Batavian Republic.  Based on the terms of the treaty, the Batavian Republic ceded to France the territories of Maastricht, Venlo, and Zeelandic Flanders.  Moreover, the accord established a defensive alliance between the two nations.  Furthermore, the Dutch agreed to pay an indemnity of 100 million guilders for their part in the war of the First Coalition, and to provide the French Republic a large loan against a low rate of interest.  The "barrier forts" in the former Austrian Netherlands were dismantled.  The port of Flushing was to be placed under a co-dominion.  Finally, in a secret clause, the Dutch agreed to pay for a French army of occupation of 25,000 till the war was ended. The 293rd Rifle Division began service as a standard Red Army rifle division shortly after the German invasion.  It served in the defense of Kiev, and managed to escape encirclement, spending the winter along the front near Kursk.  In the early summer of 1942 the division fought along the Don River against the German offensive, and after rebuilding in the Battle of Stalingrad, where it played a leading role in the encirclement and destruction of German Sixth Army, for which it was raised to Guards status as the 66th Guards Rifle Division as the battle was still ongoing.  A second 293rd was raised a few months later and fought briefly against the Japanese in Manchuria in August 1945. The Pearl River, also known by its Chinese name Zhujiang and formerly often known as the Canton River , is an extensive river system in southern China.  The name "Pearl River" is also often used as a catch-all for the watersheds of the Xi ("West"), Bei ("North"), and Dong ("East") rivers of Guangdong.  These rivers are all considered tributaries of the Pearl River because they share a common delta, the Pearl River Delta.  Measured from the farthest reaches of the Xi River, the Pearl River system is China's third-longest river, 2400 km , after the Yangtze River and the Yellow River, and second largest by volume, after the Yangtze.  The 409480 km2 Pearl River Basin (珠江流域 ) drains the majority of Liangguang (Guangdong and Guangxi provinces), as well as parts of Yunnan, Guizhou, Hunan and Jiangxi in China; it also drains northern parts of Vietnam's Northeast Cao Bằng and Lạng Sơn provinces. The La Plata skate (Atlantoraja platana) is a species of fish in Rajidae family.  It lives off the coast of Brazil, Uruguay and Argentina in depths ranging from 19 to 181 meters.  Its total length is about 60 centimeters.  Males reach maturity when 86% of total length which means about 47 to 53 centimeters.  Females attain maturity at 94% of total length, 45 to 50 centimeters.  It is oviparous, lays oblong egg capsules with horned corners.  Feeds small fish, penaeid crustaceans and cephalopods.  It is relative rare with population trend decreasing, major threat being fishing, although it is mainly a bycatch. Roads in Serbia are the backbone of its transportation system and constitute important transit roads in Europe.  Total length of roads in the country is 45,419 km, and they are categorized as "state roads" (with total length of 16,179 km) or "municipal roads" (with total length of 23,780 km). The Battle of Athens was an American Civil War skirmish that took place in northeast Missouri in 1861 near present Revere and southeast Iowa along the Des Moines River across from Croton (3 miles southeast of Farmington).  The Union victory has the distinction of being the most northerly of Civil War Battles fought west of the Mississippi, and also of being the only such battle fought along the Iowa border. The Battle of Irtysh River or Battle of Yexi River was a battle in 657 between Tang Dynasty general Su Dingfang and the Western Turkic Khaganate qaghan Ashina Helu during the Tang campaign against the Western Turks.  It was fought along the Irtysh River near the Altai Mountains.  Helu's forces, consisting of 100,000 cavalry, were ambushed by Su as Helu chased decoy Tang troops that Su had deployed.  Helu was defeated during Su's surprise attack, and lost most of his soldiers.  Turkic tribes loyal to Helu surrendered, and the retreating Helu was captured the next day.
Answer: 2400 km