I wouldn't suggest crossing them.
From the release:
Water features have played a major part in battles throughout history. Both streams and mighty rivers are natural obstacles that are sure to influence the strategy of any commander intent on victory. Battlefield in a Box provides everything you need to construct your battlefield, at all scales of gaming. Combine this set with the others from the list below to build more complex waterways.
Included in the box:
• 6x Individual 12"/30cm Stream sections.
• 1x Packet of GFS001 Green Grass GF9 Grass.
• 1x Packet of GFS017 Meadow Blend GF9 Grass.
Suitable for use with 10 – 15mm and 25mm – 35mm figure scales.