Final answer:
Changes in moisture levels can cause damage to trench stability through debris flows or mud flows when slope materials become saturated with water or through decreased growth when soils become flooded.
Step-by-step explanation:
An increase or a decrease in moisture can both damage trench stability. One way this can happen is when slope materials become saturated with water, leading to debris flows or mud flows. These flows can pick up rocks, mud, trees, houses, and cars, causing blockages and flooding. Another way is through reduced evaporation rates due to increased humidity, which can lead to decreased growth if the soils become flooded.