This is an example of *historical present*, which is more common in Japanese than in English. Please see [this][1] for details. Here, present tense is used to describe what's happening in the video vividly. Something like this is especially common in sport news, so I think you can guess how it feels. It would not have been wrong at all to use the past tense consistently.

Note that ～てくる is not "future tense". This くる is simply describing the water is physically coming toward the river/camera. See [this][2].

Regarding そこに, this そこ refers to "the current scene (calm river)", and this に is a direction/destination marker rather than a simple location marker. Here, そこに is an adverbial expression that describes something/someone entering the scene. I don't think this is a conjunction.


  [1]: https://japanese.stackexchange.com/a/73870/5010
  [2]: https://japanese.stackexchange.com/q/676/5010