Which is the more important agent of erosion, running water or glaciers and why?

Answer 1

There need to be some critical assumptions made before this one can be answered. First, how long and how much running water are you talking about? And when are the glaciers, during an ice age or not?

The Mississippi river dumps millions of tons of fertile top soil into its delta each year. And the MIssissippi has been doing this for over 5,000 years. Other rivers, like the Po and Yellow, have been doing the same kind of thing. Check this out:

"It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea, the Huang He [Yellow River] 796 million tons, and the Po River in Italy 67 million tons." [See source.]

On the other hand, during the last ice age, glaciers pushed millions of tons of top soil into North America and Europe. In fact, I was raised in Ohio where the so-called "Black Swamp" in the north-west part of the state was the result of glaciers dumping top soil (alluvium) that came from Canada.

When Earth is in an ice age and glaciers cover more than the 10% of the surface they now cover, glaciers can cause more erosion than colocated rivers can. Check this:

"The erosion rates documented in the study suggest that glaciers eroded the mountains six times faster than rivers and landslides had before glaciation began. The researchers also found that glaciers scraped at least 2 kilometers (about 1.2 miles) of rock from the mountains." [See source.]

The above refers only to coastal areas in British Columbia, Canada. However, it does indicate that glaciers move more soil and such than rivers would move over the same area. The deal here of course, is that, even in the ice ages, glaciers covered less area than rivers drained. For example, the MIssissippi drains about a third of the U.S. through itself and its tributaries like the Missouri and Ohio Rivers. And it continues to do this, while glaciers cover only about 10% of the Earth.

Bottom line, running water covers wider areas, but carries less alluvia than the glaciers. And glaciers cover less area, but carry more alluvia than the rivers over a comparable area.

Answer 2

Water Moving waters, such as rivers and streams, are the most important natural agents of erosion. Whenever it rains, water washes over the land, picks up loose soil, and carries it away. Water also washes weathered rocks (rocks altered through a process called weathering) into rivers and streams.