尿道的作用当然是让一条河流通过它。
The purpose of the urethra is certainly to have a river run through it.
大峡谷是河流侵蚀作用的产物。
山区河流发育的阶梯—深潭系统具有显著的生态学作用。
The development of step-pool system plays an important role in stream ecology.
河流水文特征，决定河流地质作用的主要时期；
Hydrological characteristics decide the main period of the river geological process.
河流水文特征，决定河流地质作用的主要时期；
Hydrological characteristics decide the main period of the river geological process.
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