Unfortunately there isn't enough information.
Check out the diagram below. We have segment BC equal to 120 meters long. Points B, C, D and E are all on the edge of the same circle. According to the inscribed angle theorem, angles BDC and BEC are congruent. This shows that the surveyor could be at points D or E, or the surveyor could be anywhere on the circle. There are infinitely many locations for the surveyor to be at, which leads to infinitely many possible widths of this canal.
