Hi @Pajtim 

Is this question specific to the Tree Bloom Change Detection exercise on Planet University or about your own work? I ask because of the tags. Thanks! 

Is my own work. Im doing LULC classification in Qgis, but how i say before, in some place the river is covered by trees.

I would recommend taking a look through the Tree Bloom Change Detection exercise on Planet University. Specifically the NDVI and Classification sections which are under the lesson titled “Tree Bloom”.
This exercise is for ArcGIS however the same principles/ processes can be applied within QGIS. 

Thanks for the answer. Can u maybe help me how can i identify water in this region? I have 3m resolution satellite images!