Final answer:
The Rio Grande's ranking as the fourth or fifth longest river in North America depends on the criteria used to measure river length, which may include or exclude tributaries and account for changes in the river course.
Step-by-step explanation:
The reason we say the Rio Grande is 'either the fourth or fifth longest river system in North America' is due to variations in how river lengths are measured. Essentially, river lengths can vary based on factors such as the inclusion or exclusion of tributaries or changes in the river course due to natural events such as flooding or human activities, such as dam construction. Therefore, depending on the criteria used, the Rio Grande could be classified as either the fourth or fifth longest river in North America.
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