"Does this river have a bridge?"
Okay, so the literal English translation is more like "Is there a bridge at this river?" So この川に is "at this river" and はしはありますか is "is there a bridge." The に particle indicates placement, and the は particle is more like a subject marker.
The は and が thing is tricky. It really depends on the situation. Here, it is best to use は。
They are similar 橋 (haSHI) (Bridge) 箸 (HAshi) (Chopsticks) From https://blog.skritter.com/2016/03/japanese-pitch-accent/#:~:text=As%20an%20example%2C%20the%20word,in%20concept%20to%20be%20understandable.