Table 3. SIMPER analysis identifying six most important OTUs contributing to the difference in community composition before (May-14 June) and after (15 June–6 July) the intrusion of Columbia River water into three shallow wells.

Taxa	2–26 (10 m)		2–29 (10 m)		3–30 (10 m)		AvDiss	Contrib%
	May–14 June	15 June–6 July	May–14 June	15 June–6 July	May–14 June	15 June–6 July
OTUs with increased abundance after river water arrival
ACK-M1 (Actinobacteria)	0	8.6	0	9.6	0	2.4	3.4	4.8
Chloracidobacteria	0.6	2.6	2.6	6.6	2	4.2	1.5	2.2
CL500-29 (Actinobacteria)	0	2.8	0	2.4	0	1	1.0	1.4
OP3	1	7.4	0.4	4.6	1.8	7.4	2.8	4.0
Sphingobacterium	1.8	3.6	4.2	4.8	1.8	2.8	1.2	1.8
ZB2	0	0	3.6	6	1	2	2.0	2.7
OTUs with decreased abundance after river water arrival
Aquabacterium	2.8	1.4	2.2	1.2	2	1.4	1.0	1.5
Clostridium	5.6	0.2	10.6	0.8	0	0	3.9	5.4
Emticicia	2.2	0	8	1.8	0	0	2.3	3.2
Limnohabitans	0	0	2.2	0.2	5.2	0.4	1.7	2.7
Pseudomonas	4.6	0.6	1.2	0.4	3	1	1.5	2.2
Zoogloea	6.8	0.4	13.6	0.6	0	0	4.9	6.7

Abbreviations: AvDiss, the average of the Bray–Curtis dissimilarity between all pairs of samples; Contrib%, the contribution of this OTU to the total dissimilarity between communities before and after the river water intrusion; OTU, operational taxonomic units. The gradual shift of these OTUs in time are graphically illustrated in Supplementary Figure S5.