What is the purpose of analyzing the bed-load of a river?
Typically rivers can be divided into three areas when it comes to erosion:


far upstream there is the area where most erosion happens
in the middle mostly transport of material happens
downstream rivers mostly deposit material


Which also can be determined by the shape and size of rocks in a streambed, which correlate directly to erosional force at that point of the river.


larger, rough rocks are rather freshly broken off the bedrock, it takes a lot of force to move these rocks, ergo there is a lot of erosion happening, due to a combination of inclines, amount of water and shape of the riverbed
smooth, smaller pebbles have been in the river for a longer time. They usually have been transported some distance from where they came into the riverbed, which wore them down to their current form. It also takes considerably less force to push/roll a smallish pebble compared to rough chunks of rock. Ergo the erosion at this point of the river is smaller
sand and silt (which still are some form of rock) are typical riverbeds close to the rivers mouth. It takes ages to grind rocks to sand, and it is much easier to transport sand over long distances, so when the sand drops to the riverbed, the erosional force is close to nothing.


For some more information, look here e.g.: http://www.alevelgeography.com/the-long-profile-changing-processes-types-of-erosion-transportation-and-deposition/