Modify River behavior

I would like to modify a default Elgg behavior, but I do not know how to do it.

I want the main screen of my social network (the River or Activity) to be more complex. Something similar to what Facebook offers. For this I must understand better how the view 'River' works and that functions or checks are being done to display the contents. Could someone tell me where to find this information?

An example could be would not show all public content, just some. Since if many users create public content, they could quickly go to the bottom.

Thanks all