The Russ themselves told Constantine Porphyrogenitus that they were not Slavs

In the undated part of the "Primary Chronicle" Nestor first attributed "Rus" to the Varangians, and then wrote that they were special Varangians – not "svie", that is, not the Swedes, not "urmans" - not the Normans-Norwegians, not "anglians", that is, not the angles, and not "Goths" - not the inhabitants of Gotland. With his explanation, the chronicler wanted to bring additional clarity to an important ethnographic issue, but in fact, everything was only confused. 

Among the reading and writing brethren there were many who wanted to prove the Slavic origin of early Russia, that is, those chronicled Varangians who came to us either in 852, or in 862. (the chronicler himself got confused in the Dating). The chemist and astronomer Mikhail Lomonosov was the first to stir up the water. His most prominent followers were: Director of the Imperial theaters S. A. Gedeonov and a major specialist in flies, entomologist S. Ya. Paramonov (literary pseudonym Sergey Lesnoy). They were still historians, but they were ideologically correct. Their leavened patriotism was enough to inspire hundreds of thousands of empty heads that they were maliciously deceived by slanderers and haters of the Russian people, calling the Varangian Russ Normans. In the late Soviet Union, when the ruling gerontocracy needed a strong Patriotic bond, the anti-Normanist theory acquired the unspoken status of an official point of view, and no academic dared to Express even an indirect doubt about it without a thousand reservations. 

The Byzantine Emperor Constantine Porphyrogenitus, not at all suspecting what kind of pig he was laying for future anti-Normans, decided to write a work "on the management of the Empire" in the middle of the 10th century. In the course of working on this essay the Emperor was gathering information from a variety of sources, and when possible, talked with travelers and eyewitnesses. One day he had an interview with one or more Rus merchants who had come to Constantinople on commercial business.

Konstantin was interested in how Russ overcome the Dnieper rapids. The merchants told the story, and the Emperor (or his Secretary) wrote it down. As a result, it turned out that the rapids on the Dnieper had two parallel names – Russian and Slavic, which was recorded in the book. In Russian the rapids was called: Assoupi, Ulvari, HALANDRI, Aitor, Vorovoro, Leandi, Strucon. In Slavic, their names sounded much more familiar to us: Do not sleep, Ostrovuniprach, which means "Island of the threshold", the Noise of the threshold, Neasit, Vulniprach, Veruchi, which means "Boiling water", Naprezi, which translates as "Small threshold".

Historians and linguists still argue about particulars, but in General in professional circles there is no doubt that the "Slavic" names of thresholds are old Slavic, and the "Russian" names with the highest degree of probability belong to the old Norse or old Swedish language. Not everyone agrees that Konstantin Bagryanorodny personally met with Russians, referring to the fact that for Russian merchants, his informants were too poorly versed in the geography of the upper Dnieper. Well, maybe someone else told the Emperor about the rapids. But this does not make the Scandinavian names of the rapids Slavic. The ancient Russ spoke Swedish in spite of the opinions of Nestor, Lomonosov, and Gedeonov. However, in the second generation they switched to the vernacular old Slavic, and for them the problem of normanism disappeared like smoke.