From early 1990’s until early 2000’s major river regulation measures have been conducted in the river Kalajoki to improve flood control, hydro power and log floating. These measures significantly changed morphological and hydrological conditions in the river. The main measures included regulation of the Kalajoki in 1903–1910, embankment of Niemelänkylä area in 1955–1960, water management plan of the river Kalajoki in 1960’s and 1970’s, regulation of the middle part of the Kalajoki in 1967 – 1988, flood control of Alavieska area in 1990–1992 and restoration of the Juurikoski in 2004–2005. Since late 1970’s, the court order incorporated studies on effects of regulation measures on water quality, fish stocks and fishing into the rulings. Furthermore, court ordered compensation measures like fish stockings and rehabilitation measures. Permits also included orders to observe the success of the compensation measures. The government has been the major actor in almost all water management projects in the river Kalajoki. Furthermore, the government holds all water level regulation permits in the Kalajoki area. Hence, the government has been responsible for studies and compensation measures included in the water management projects. The most significant compensation measures have been stocking of 4.7 million one summer old whitefish in 1981 – 2010, restoration of the river in 1999 – 2004 to improve reproduction conditions for crayfish and lamprey and stocking of 30 000 mature crayfish in 2003 – 2006. This report is an overview of the studies carried out in 1978–2010. Based on these studies, the effects of the river regulation measures, water level regulation and compensation measures on crayfish, lamprey, fish stocks and the river Kalajoki as a habitat of aquatic organisms were evaluated. In addition, the available data has been used to evaluate the effects of other factors on crayfish, lamprey and fish stocks.