Drifts can be driven by hydraulics (passive drift), invertebrate life stage and/or behaviour (active drift). Drift estimations can be derived qualitatively inferred by hydraulic conditions, diurnal periodicity, and taxa-specific traits. Season is also an important component, for example, ice melting (or freshet) is known to significantly increase flows. Predator avoidance can be another causative factor for the drift, for example, does the time of the drift coincide with fish hatching events? Hope this helps. 

Thanks a lot for your answer Jorgelina. In this case the amount of drift was incredibly different from 1 night to another. The first night of experiment we measured "normal" levels of drift; the second night we measured massive drift (1000-fold higher compared to the night before); the third night and the following 3 weeks we measured again "normal" drift rates, comparable to the first night of experiment. Do you think that this 1-night event might be caused by a localized event upstream? Ice melting? Hydraulic modifications (anthropically induced)?

Hi Alessandro, it is hard to say what can be the cause for the drift. However, many factors (as the ones I mentioned in my previous reply) can happen fast. Is there any way you can find flow information? or if there is increased fish presence? Definitively anthropogenic modifications can be also a causative factor (maybe construction or drilling in the area?). 

Hi Alessandro,

did you monitor the water temperature and the discharge? Sudden changes in one or both of those parameters could cause an increased drift as well.

dear collegue

According to my experience, seeing the big difference of drift between the previous night and the next, I suspect two events:

- an sudden abnormal water release from some retention structure (dam etc.)

- a spillage of toxic substance during the night

Maybe you can check out these events

cheerio