The Secret History of the Mongols (1240 AD) frequently mentions a "Black Forest of the Tuul River" where the palace of Ong Khan was located.

Its water runs through the southern part of the capital city of Mongolia, continuing in a western direction in large loops.

In addition, the steady influx of people settling near the river may be causing a degradation of water quality.

[4]Monsieur de Bourboulon (Minister of France) also visited the river in 1860: Monsieur de Bourboulon, wanting to stroll without escort through the Mongol capital, made his way early past the Russian quarter to the banks of the Toula…Monsieur de Bourboulon came to the banks of the Toula and sat on a mound underneath a birch tree: in ecstasy before this magnificent landscape, one of the most beautiful given to man to contemplate, happy to see the pure waters of this beautiful river murmuring at his feet, and rejoicing avidly, after the arid and monotonous deserts that he crossed, at the fertility of this rich and picturesque nature.

He was there plunged in a delicious reverie when he heard the confused noise of words behind him: it was Mongols of the high class, Taiji or gentlemen, who were trying to make him understand through various signs that if he wanted to cross the river they would gladly take him on their horses.

[5]Orkhon inscriptions states that Once Toquz Oghuz were living on the sides of Tuul River neighbouring to Tatars.