African Wildlife & Environment Issue 77 FINAL ISSUE

CONSERVATION

blocking the estuary, manifest as water of different colours precisely at the halocline. However, the aquifer is flowing freely into the ocean. Here we have two actions occurring simultaneously. The wave action of the ocean is constantly pounding

But what happens when the freshwater flow is from an aquifer rather than a river? I have been fascinated by this concept for some time and have been scouring the satellite imagery made available by Google Earth. In this article I present some of the images I have found to be representative of the interface between freshwater flows from an aquifer and saline water along a beach. Adjacent to the Groot Brak River estuary, eastwards towards Wilderness, there is a significant mountain range close to the coast. The Outeniqua mountains intercept the warm moist wind blowing from the ocean and force precipitation to occur. This rainfall and fog permeate a coastal aquifer filled with alluvium and is thus highly porous. Where the aquifer meets the sea there is a pressure gradient driving the natural flow of freshwater from the mountain, through the aquifer, into the adjacent ocean. This flow is different to a river, which is a single precise point of discharge. In the case of a coastal aquifer, particularly one made up mostly of alluvium, that flow will be diffuse along a wide front.This is fundamentally different to that of a river. Image 1 shows all these elements and orientates the reader. Noteworthy in this specific image, taken by satellite on 9 July 2004, is the following. The estuary is closed because of low river flow. There is some sub-surface flow through the sandbar

the beach, but the pressure gradient from the mountain is forcing freshwater into the ocean all along the beach. This freshwater floats above the seawater. Freshwater doesn’t create foam and is not inclined to produce the same waveform that saltwater produces, so these freshwater flows are clearly evident as 'L' shaped plumes of flat water running parallel to the beach, and then breaking through the surf line in clearly demarcated places. This pattern is particular to the contact between fresh and saltwater at diffuse sites from a coastal aquifer. The trained eye can quickly identify these unique 'fingerprints' from any satellite image. In Image 2 we have a partially open estuary, but most of the normal (reduced) flow from the river is forced to move through the sandbar.This resembles the flow from an aquifer, with classic 'L' shaped plumes through the surf line. These plumes are clearly visible from the diffuse aquifer flow along the entire beach east of the estuary. Note the clear separation between the backline of Image 2: Satellite image taken on 21 August 2015 showing surface flow from a partially open estuary adjacent to active diffuse flows from the coastal aquifer. Note the classic 'L' shaped plumes penetrating the surf line always accompanied by a clearly visible halocline. Image courtesy of Google Earth.

Image 3: Satellite image taken on 3 July 2019 shows no visible freshwater discharge from the estuary but suggests the presence of sub-surface flows through the alluvial fan of a larger pre-existing estuary used to include Southern Cross and The Island. Image courtesy of Google Earth.

17 | African Wildlife & Environment | Issue 77 (2020)