The EPA in Ireland in their studies of plastic in the fresh water environment showed how the water slows down as it reaches the lower courses and so many of the plastic nurdles are deposited here, as shown above.
These are hard enough to pick up from sand but if they are in mud you would need a beak to pick them up.
This is a picture of them in clusters in the water. They look like fish eggs and so fish and birds eat them.
These plastic pellets attract persistant organic pollutants . They are called persistent because, like the pellets they don’t disapear over time . They are mostly endocrine disruptors and affect fish, birds and mammals ability to breed and to lactate.