Canals branch out across the plain.

Literal

Plain [object-を] crossing waterway [subject-が] branching is.

平野 is 'plain/flatland.' 水路 is 'waterway/canal.' 枝分かれする means 'to branch out' — 枝 (branch) + 分かれる (to split). を横切って means 'crossing.' A geography/landscape description sentence.