It is important to determine the water quality of the creeks in the Lake Sapanca basin since it is used for drinking water supply. For this purpose, environmental parameters and diatom assemblages as biological quality components were investigated to determine the ecological status of eight creeks with monthly intervals between March 2015 and February 2016. A total of 132 diatom taxa were identified in eight creeks of the Lake Sapanca basin during the studied period. 19 taxa increased their relative abundance higher than 30% in at least one sample and showed a different seasonal pattern. Environmental parameters [nitrate-nitrogen, orthophosphate, total phosphorus (TP), specific conductance, temperature, pH, and dissolved oxygen] were effective on the distribution of diatom assemblages. The ecological status of the creeks was characterized by using four different diatom indices (TIT, EPI-D, IPS, and TDI), however, only Trophic Index Turkey (TIT) represented significant positive correlations with log (TP) gradient. Both environmental parameters, the relative abundance of indicator species, and TIT index results stated that 1st, 2nd, 3rd, 5th, 7th, and 8th stations had moderate, while 4th and 6th stations had good water quality. The good correlation of TIT index with log (TP), and its compatibility with the presence of indicator species at different stations indicates that this index could be used in the Lake Sapanca basin, and Sakarya basin where the lake basin is located.