Sampling sites (61) along the entire course of the Sava River, with five major sections. Inset map shows eight very close sites at the beginning of the Middle Sava section, with two sites marked by arrows where the upstream front of the invasive amphipod Dikerogammarus haemobaphes was detected in 2011 and in 2015. Large black dots represent 15 sites were natural and artificial substrates were sampled separately.