Question

I know what the caged bird feels, alas!


When the sun is bright on the upland slopes;


When the wind stirs soft through the springing grass,


And the river flows like a stream of glass;


When the first bird sings and the first bud opes, 5


And the faint perfume from its chalice steals -


I know what the caged bird feels!





To what other object is the river being compared?

sun

grass

glass

perfume

Answer 1

The answer to your question is in this line here: "And the river flows like a stream of glass;"
Your answer is glass. :)

Answer 2

Remember, when comparing two things, "like" is a word that is often a dead give-away.