Welcome Iniptia, and nice find?
You didn't say how big a river or show the larger samples. Take one of the larger specimens, weigh it and immerse it in a known volume of water. Measure the volume of the water displaced from the container. Now
divide the weight by the volume and you have specific gravity or density. Look up a density table for common metal alloys and their specific gravity. Large globs of lead can have a number origins. sailors on a large ship made their own musket balls. Lead was often used to stabilize the ship's keel. Lead was melted in large quantities to seal plumbing joints till plastic soil pipe came along.
Good luck in your efforts!