Passage I

Two measures of water quality are the number of Escherichia coli bacteria present and the biotic index, BI (a numerical value based on the type，diversity, and pollution tolerance of aquatic invertebrate animals). Both of these measures can be affected-by water flow.

E.coli levels that are above 100 colonies formed per 100 mL of water indicate reduced water quality. Figure 1 shows the E. coli levels on 5 collection days at Sites l and 2 in a river.

Table 1 shows how water quality rating varies with BI. Table 2 shows the average BI of each site daring the collection period.

Figure 2 shows the water flow at each site on the 5 collection days.