Old developed systems of agricultural terraces are found in settled areas with high relief in different parts of the world. The present trend to abandon many of these terraced areas constitutes a process that increases erosion and sediment yield values following the collapse of supporting walls. This paper addresses the problem of changing human activities in the fragile environment of the historical terraces in the Central Andean mountains of Peru. The study is based on field experiments. Eight small plots were installed in the Santa Eulalia basin at altitudes of 2800 m and up to 3650 m. Annual run-off coefficient values were less than 5% and sediment yield values less than 1 g/m² on the experimental plots. Daily rainfall intensity does not exceed 10 mm/d on most rainy days. Simulation of rainstorms by sprinklers was performed on terraces with different physiographic characteristics, lithology, soil, exposure, slope, altitude, degree of abandonment, and vegetation cover. Rainfall simulation tests revealed that run-off is high on steep, nonvegetated slopes and very low on grass-covered, low-angle slope terraces. A morphometric analysis was conducted on about 300 terraces with the same physiographic parameters. The average terrace area was 170 m², and most terraces surveyed were in a semiabandoned stage. Terrace degradation was noticeable by wall swelling, collapse, and deterioration of wall and terrace structure. Terrace degradation is a function of physical, economic, and social processes, which are linked and irreversible.