Why do the Swakop and Kuiseb rivers usually disappear in the Namib Desert?

The Swakop and Kuiseb rivers, like many watercourses in the Namib Desert, typically disappear because the region is an arid, almost rainless area with a poorly developed drainage pattern.1

These rivers originate on the interior plateau and descend the western escarpment, but they are ephemeral, meaning they only flow on the surface following heavy rainfall in the interior. Under normal conditions, they terminate in the desert, where they either die out or end in a vlei (a salt pan or mud flat) against or among the sand dunes. Additionally, a portion of the water from these major streams seeps into the sands of the streambeds, creating an underflow that is sometimes tapped for water supplies. Only during rare flood years do these rivers possess enough volume to reach the Atlantic Ocean at Swakopmund and Walvis Bay.123