The procedures of expertise for river and canal works were not fully standardized in 18th-century France, unlike some regions of Europe, such as England and Duchy of Milan, where recourse to expertise had become a normal procedure during the course of the 16th century. The aim of this talk is to show how this lack of decisive administrative structure of expertise in France finally contributed to the generation of a new role for scientists in the expertise of public works in the latter half of the 18th century. It was in the 1770s that the Royal Academy of Sciences of Paris got involved in some of the canal construction works, as a sort of technological consultant. However, things did not proceed in a straight line. Though the Academy was an institution mainly specializing in scientific research, it had members of different professional backgrounds, and there existed rivalries between some of them to determine who led the expertise of the projects in question. On one hand, there were members with civil engineering skills and institutional backgrounds, like J. R. Perronet of the Corps des Ponts et Chaussées and his subordinate engineers; on the other hand, there were academician scientists who specialized in mathematical sciences (especially in theoretical mechanics), like D’Alembert and Condorcet. In the 1770s, it was the former who were chosen by the Academy to lead the expertise, mainly by their experience as engineers in supervising road and bridge construction. By contrast, the latter, the savants, were not able to be involved in those kinds of issues by official pathway, and to do so, they used their connections with A. R. J. Turgot. In the 1780s, however, we see the apparition of a new division of works between the mathematical scientists and the engineers, especially in the case of the Canal of Yvette. After the proposal of a new plan for this project by Defer de la Nouerre, an engineer, the rivalry shifted to the two engineers who had different conceptions. In this situation, academician scientists, especially mathematicians, took the role of judges of opposing plans. Under the Calonne’s administration, which promoted the engagement of scholars’ commissions in the decision-making processes in general, the academician scientists seized the opportunity to consolidate their roles as scientific experts by their ability to mobilise several domains of scientific knowledge.