Well the original mapper clearly didn't have lots of time...
Maybe it was part of an import project at a low resolution?
@thinkMoult I haven't checked. But I don't think so. Other rivers in this region are mapped in detail. It also stopped at an arbitrary point
Ouch, I was like that too when I begin to map rivers 4-5 years ago. No high resolution Bing satellite layers back then — so, approximate traces it was!