Complex sentence: Its length from source to confluence with the Thames is considerably greater than that of the Thames from Thames Head; its flow is also more consistent than the winterbourne Thames, but the Churn is regarded as a tributary historically and therefore by most geography guides.
Split into simpler sentences: Its length from source to confluence with the Thames is considerably greater than the Thames from Thames Head. Its flow is also more consistent than the winterbourne Thames, but the Churn is regarded as a tributary historically and therefore by most geography guides.