A glacier is a giant mass of ice that has accumulated for many years and moves slowly from higher elevations to lower elevations and sometimes sea level. Due to its immense bulk, a glacier will take a very long amount of time to melt even when it reaches the ocean. As a result, it will continue to erode the ocean floor as it moves into the ocean. Fjords are a perfect example of erosion at and below sea level as a glacier moves into the ocean. Rivers and streams, on the other hand, are already liquid water. When the moving water from them reaches the ocean, it quickly disperses into a large, open body of water. As a result, the force of the flowing water very quickly disappears once it hits the ocean. That is why rivers and streams don't erode below sea level but glaciers do.