Tidal bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than between high and low water), and where incoming tides are funnelled into a shallow, narrowing river via a broad bay. Large bores can be particularly dangerous for shipping, but also present opportunities for river surfing. The funnel-like shape not only increases the tidal range, but it can also decrease the duration of the flood tide, down to a point where the flood appears as a sudden increase in the water level. Note the tidal bore takes place during the flood tide and never during the .