Wied tal-Kalkara and Wied ta’ Rinella form part of the drowned valley system of the Grand Harbour Area and make up the majority of the remaining natural open space in this area. For this reason these valleys are considered to be important both from the ecological and the geological point of view.
The fault system in the Grand Harbour area led to the formation of fluvial valleys such as il-Wied tal-Kalkara and il-Wied ta’ Rinella. Due to the post-Miocene (23–5 million years ago) tilting of the Maltese Islands to the North Northeast, parts of these fluvial valleys along the Northeastern coast were drowned. This is further confirmed by the continuation of the fluvial valleys in the sea bed, buried beneath deposits of silt and mud, beyond the mouth of the Harbour where the drainage once discharged its flow into the sea. These valleys constitute what is left of the emergent parts of the Grand Harbour drowned river-channel system.
The tilting of the Maltese islands to the NNE facilitated the surface water flow through these valleys during the Quaternary pluvial climates. For this reason, the Grand Harbour area is considered to be an important hydrogeological unit since it represents the major drainage system of the Quaternary age.
MEPA scheduled Wied tal-Kalkara and Wied ta’ Rinella as a Level 2 Site of Scientific Importance (Geology) and a Level 4 Area of Ecological Importance as per Government Notice No. 930/02 in the Government Gazette dated 18 October 2002.