Unraveling the Mystery: The Formation of Reverse Deltas in Lake Outflows

Unraveling the Mystery: The Formation of Reverse Deltas in Lake Outflows

We all know deltas, right? Those classic landforms where rivers meet the sea, depositing sediment to create new land. But what if I told you deltas can form in reverse, at the outflow of a lake? It sounds a bit crazy, I know, but these unusual formations, called reverse deltas or lake-outlet deltas, are a fascinating exception to the rule.

Now, think about a typical delta. A river slows down as it enters a larger body of water, like an ocean or lake. This slowdown causes it to drop its sediment, gradually building up that familiar fan shape with all those distributary channels. But at a lake’s outlet, things usually speed up! The water picks up pace as it exits the lake, meaning it can carry more sediment, not less. So, logically, you wouldn’t expect a delta to form there.

So how do these reverse deltas buck the trend? What’s the secret? Well, it all boils down to a perfect storm of hydrological conditions. They form when a lake’s outlet stream experiences a reversal of flow. Think of it like this: imagine a bully pushing you back the way you came. That’s essentially what happens when a larger river floods and overpowers the smaller outlet stream.

For this to happen, a few things need to be just right. First, there needs to be a minimal elevation difference between the lake, its outlet stream, and the larger river it flows into. This makes the outlet stream vulnerable to the backwash from the bigger river. Second, the outlet stream often joins the larger river at a sharp angle, right on the outside of a bend. During floods, water tends to pile up on that outside bend, making it even easier for the flow to reverse into the outlet. And finally, you need a good old-fashioned flood in the larger river. When that happens, the swollen river overwhelms the outlet stream, forcing water—and sediment—backwards into the lake’s outlet. Over time, this reversed flow deposits sediment, slowly but surely building a delta in reverse!

You might be wondering, “Where can I see one of these things?” Well, they’re not exactly common, but they do exist. One example is the Androscoggin Lake in Maine, which boasts a pretty impressive lake-outlet delta system.

Now, a quick note: don’t confuse these reverse deltas with inverted river deltas. Inverted deltas are a whole different beast, where the narrow end faces the sea and the wide end is inland. The Sacramento-San Joaquin River Delta in California is a prime example of that.

So, why should we care about these quirky reverse deltas? Well, they offer a valuable glimpse into the dynamic nature of our landscapes. They remind us that things aren’t always as straightforward as they seem and that even stable systems can be flipped on their head by extreme events like floods. By studying these unusual formations, scientists can gain a better understanding of the complex relationship between rivers, lakes, and the world around them. They’re a reminder that nature always has a few surprises up its sleeve!