This paper aims to discuss the existence of the so-called ”Eighty effect”. This phenomenon refers to the general decline in river flows in the Spanish headwaters until the 1980s. We chose the Tagus River Basin as a case study for several important reasons: it is subject to certain increasing demands as well as environmental objectives; it is under the constraints imposed by the Albufeira International Agreement, and it is the origin of water transfers to other basins, ensuring water supply for urban, agricultural and ecological purposes. In the context of multiple demands and potentially decreasing levels, a clear understanding of existing water supply is essential to ensure its proper management. To promote this crucial understanding, we use recognized statistical methods in economics and hydrology, such as Time Series Analysis, intervention analysis and structural change contrasts. The empirical evidence found shows clearly a level shift in flows form Tagus sources.