What is a grassed waterway primarily used for?

A grassed waterway is primarily used for conducting surface runoff. This type of natural or constructed channel, which is lined with grass or other vegetation, serves the important function of improving water quality and controlling erosion. By allowing surface water to flow through a vegetated channel, the grass helps slow down the water, allowing sediments and pollutants to settle out before the water continues downstream. This design is effective in managing stormwater runoff, particularly in agricultural and developed areas, where the concentration of surface runoff can be significant.

The effectiveness of grassed waterways in transporting surface runoff comes from their ability to handle water flow while providing stabilization through vegetation. Properly designed grassed waterways can significantly reduce the velocity of flowing water, thus minimizing the potential for soil erosion while promoting infiltration and groundwater recharge. In contrast, other options like carrying groundwater, decorative landscaping, or drainage of underground streams do not align with the primary function of a grassed waterway, as they focus on different aspects of water management and landscaping.