Figure 5.7.12 – A: Retention ponds with steep slopes and a simple liner without soil must be avoided. The slope makes it difficult for species to access or escape the pond and the lack of soil increases the chances of algal bloom and eutrophication with no macrophyte growth. B: An optimal retention pond should present a shallow zone (depth ~ 20-50 cm), allowing species to access the pond and use it as a breeding site. It can also have clay or liner and clay (min. 30 cm) in order to allow macrophytes to grow, reducing the risk of eutrophication (Source: Adapted from EGIS).