Brackish water regions provide a wide variety of environments and high biodiversity. Therefore, these regions have experienced high human activity since ancient times. Coastal environments have been changed by both nature and the human activities. Today, human activities have often caused problems, e.g. deterioration of water quality or degradation of the ecosystem. Understanding the history of interactions between human activities and the natural environment is a crucial first step for establishment of countermeasures against environmental problems. The interactions between nature and human activities around Lake Kitagata, Hokuriku district, Japan were investigated using pollen analysis and CNS elemental analysis as an example of environmental change in a brackish water region. <br>Lake Kitagata is a brackish lake and is connected to Daishoji river; it flows into the sea at the border between Fukui and Ishikawa prefectures. Human activities have been high at this location since ancient times and it is well known that the environment has been greatly modified during historical times. <br>Salt making has been a primary cause of deforestation in this area from the Nara to Heian periods (the 8^th -9^th century AD). In addition, land transformation to arable fields took place in the Heian Period (from the 11^th century AD) based on pollen analysis. Arboreal pollen decreased significantly during these times. During the period when the salt making furnaces were used, based on C/S ratio, lake water was not suitable for salt making because the lake was fresh. At this time the fresh-water lake seemed to contribute to local rice cultivation, but the lake water soon became brackish, based on C/S ratio. In consequence, rice cultivation was reduced in the area, and buckwheat started to be cultivated in the 14^th century AD. The land was deforested further, and it seems that the area supported only scattered trees. In the early Edo Period (the 17 century), pine trees were probably planted near the lake, as seen in the appearance and increase of Pinus pollen. Construction work in the Daishoji River basin during the Edo period increased the C/N ratio of lake water. The construction of the Kaiden bridge in the 19^th century did not strongly affect the lake based on C/N and C/S ratios, although it was reported that it damaged oyster cultivation. Natural disasters did not have a long-lasting influence on the environment of Lake Kitagata, but they did affect regional vegetation for short time intervals.