Rivers today have high muddy banks, sandbars and bends. But they didn’t always look that way. Because it wasn’t until the [--1--] of tree-like plants, some 330 million years ago, that rivers were corralled into their current form. Before that, ancient waters flowed wide and shallow over the land, with little to [--2--] them other than mountains. So says a study in the journal Nature Geoscience.

Researchers looked through over 400 studies of the Earth’s rock record, and visited nearly 70 field sites. And they found that channel formations in the rock—a signature of modern rivers—didn’t appear until the Carboniferous period, when tree-like plants evolved. That’s because larger plants needed deeper roots, which [--3--] river banks and forced rivers into narrower paths. And deep roots helped form sticky clays, which are harder to [--4--].

All this engineering was to the trees’ advantage, the researchers say. Because river banks provide trees with easy access to water, without the [--5--] of flooding. Pretty much what we humans want. Many of our greatest cities formed along river banks—for which we might have trees to thank.
【视听版科学小组荣誉出品】
evolution constrain stabilized erode constant risk
树木造就的现代河流 今天的河流有着高高的泥堤、沙堤和弯道。但是它们在树类植物进化前并不是这样子的。大概33亿年前，河流才开始有它们现在的这种样子。在那之前，古老的水域很宽，而且很浅，除了山之外没有什么可以限制它们。引自《自然地球科学杂志》。 研究人员查阅了400多份地球岩石记录，并走访了近70个研究现场。研究发现岩石中河道的形成，也就是现代河流的迹象直到石炭纪（也就是树类植物进化后）才开始出现。因为越大的植物需要扎根更深，这就使得它们需要巩固河岸，强迫河道变得更窄。这些深深的根系促成了不易腐蚀的黏土的形成。 研究人员认为，这一切的工程都是树木的功劳。因为河岸为树木提供便利的水源，且洪水泛滥的可能性小，这也正是我们人类想要的。许多我们最好的城市都是沿江的。我们需要感谢树木为我们带来的这些。