Database: mondial_geo
Question: What proportion of rivers have a length of more than 3,000 miles? Please provide the name of a Russian river that is more than 3,000 miles long.
SQL: SELECT CAST(SUM(CASE WHEN T1.Length > 3000 THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.Name) FROM river AS T1 INNER JOIN located AS T2 ON T1.Name = T2.River INNER JOIN country AS T3 ON T3.Code = T2.Country