it is not really clear what you want to obtain. Also, if you use a for-loop there is no need to increment at the end as this is done automatically (this would be required if you would be using a for-loop also you are assiging the st.order and then re-assign it in your code maybe you are trying to do this?
st.order = input('stream order=');
for ii = 1:st.order
st.number(ii) = 0;
st.length(ii) = 0;
...
end
but again, it is not clear what you are trying to achieve, and most of all, if the structure you design is the appriorate way to go (in your tag you placed matrices but you do not put your data in a matrix but in a structure)
