But catching salmon in deep water is not that easy and the cubs have lots to learn. The annual arrival of spawning salmon brings huge quantities of food into these high rivers that normally struggle to support much life. Although relatively lifeless, the power of these upland rivers to shape the landscape is greater than any other stage in a river's life. Driven by gravity, they're the most erosive forces on the planet. For the past 5 million years Arizona's Colorado river has eaten away at the desert's sandstone to create a gigantic canyon. It's over a mile deep and at its widest it's 17 miles across. The Grand Canyon. This river has cut the world's longest canyon system - a 1,000-mile scar clearly visible from space.


在深水里捕捉鲑鱼可不是一件容易的事。这些小熊还有很多本领要学。一年一度的鲑鱼产卵季节给这些河流带来丰富的食物来源。通常情况下这些河流很难让太多的动物生存。虽然生物种类不多，但这些上游河段水位落差大，改变地貌的力量大于其它河段。受重力的影响，水流通常是地球上最主要的侵蚀力。过去500万年中，亚利桑那州科罗拉多河冲走大量的荒漠沙石，冲刷出了一条巨大峡谷。它深达1600多米，最宽的地方超过27公里。这就是科罗拉多大峡谷。这条河流切割出了世界上最长的峡谷，它长达1600多公里。